The primary purpose of classification is to facilitate the retrieval of technical subject matter. In order to reliably retrieve technical subject matter using the CPC classification system it is important for all technical subject matter to be consistently classified.

CPC has its own classification rules which appear in the scheme, definitions, or both. See MPEP § 905.02. In the absence of any specified classification rules in the scheme and definitions, classification practice follows the rules used in the IPC, as set forth in the IPC Guide. The IPC Guide is available at www.wipo.int/classifications/ipc/en/.

The CPC database maintains technical information regarding the patent family documents for each patent document included. Patent documents contain two types of technical information to classify. These are invention information and additional information. All of the symbols which are allocated to an application, in accordance with CPC guidance, define the application's classification picture.

I. PATENT FAMILY

A patent family is a group of documents related to each other by common priorities. Examples include a pre-grant publication (PGPub) of an application and any patent issued from the same application. A parent and divisional application also are considered as being in the same family.

In the CPC database, CPC symbols are associated with patent families. When a document is classified in CPC, the symbol associated with the document is stored in the CPC database as an association between the symbol and patent family in which the document is a member. This means that whenever a CPC symbol is allocated to a document all the documents in the family receive the same symbol.

In the CPC database, a document cannot be classified independently of the other documents belonging to the same patent family. Every classification symbol associated with a document, i.e., allocated to the document, is associated with every patent document in the family. Each allocation of a symbol to a patent family has additional attributes stored in the CPC database. These include the following:

1. Classification symbol
2. Type (Invention/other additional information)
3. Position (First/later classification)

II. INVENTION INFORMATION (OBLIGATORY/MANDATORY)

All disclosed invention information in a patent document must be classified in CPC in order for a user to retrieve the invention information from the system. Therefore, it is mandatory to classify all the invention information disclosed in each family to be classified.

Invention information is technical information in the total disclosure of a patent document (for example, description, drawings, claims) that represents an addition to the state of the art. The invention information is determined in the context of the state of the art, using guidance provided by the claims of the patent document, with due regard given to the description and the drawings. "Addition to the state of the art" means all novel and unobvious subject matter specifically disclosed in a patent document, which advances the state of the art, i.e., the technical subject matter disclosed that is not already in the public domain.

Patent documents should not be classified as a single entity. Rather, all different inventive entities, claimed or disclosed within the patent document, should be identified and separately classified. Such different inventive entities are represented by different claims, alternative variants or different categories of subject matter (for example, a product and a method of its production).

III. ADDITIONAL INFORMATION (DISCRETIONARY)

Additional information is non-trivial technical information which does not in itself represent an addition to the state of the art but might constitute useful information for the searcher. The additional information complements the invention information by identifying the constituents of a composition or mixture; elements or components of a process or structure; or use or applications of classified technical subjects.

Unlike invention information which must be classified, any additional non-trivial technical information that would be useful for search is classified on a discretionary basis.

IV. C-STAR (C*) INFORMATION (OBLIGATORY/MANDATORY)

A C-STAR (C*) is applied to an allocated symbol from the classification picture on an application when that symbol represents at least one concept that is claimed. C* designations should be applied to symbols regardless of type or position (any of family, invention, and additional). No new CPC symbols should be allocated merely to address C*s. Narrow/specific CPC symbols that fall within the claimed subject matter shall be designated as C*. Narrowly allocated symbols that overlap with broadly defined claims shall be designated as C*. There is no limit to the number of the CPC symbols within the classification picture of an application that can be designated as a C*, but at least one CPC symbol must be designated as a C*.

A. Type Attributes

Every classification allocation has a "type" attribute. There are two values that this attribute may have:

1. (A) Invention Information
2. When allocating CPC symbols to a document based on disclosed invention information, the type attribute is set to "invention." This is typically done by putting the symbol being allocated in the "mandatory" section of the classification form.
3. (B) Additional information
4. When allocating CPC symbols to a document based on disclosed information that is not inventive, the type attribute is set to "additional". This is typically done by putting the symbol being allocated in the "discretionary" section of the classification form.
5. (C) Position Attribute
   1. First - Each patent family will possess a first-listed CPC classification symbol which is selected from the required classifications for the inventive subject matter of the patent family. This "first" CPC classification is the inventive classification symbol which most adequately represents the invention as a whole for the patent family. In situations where a later-published family member possesses a different first-listed CPC classification, this will override the previous first-listed classification (which will be retained for the patent family as an inventive classification symbol). There is one and only one "first" position attribute per patent family. The first attribute is associated with the invention symbol that most completely covers the technical subject matter of the disclosed invention. The first position symbol is identified as the first mandatory symbol listed on the classification form.
   2. Later - If a symbol allocation is not the "first" position symbol, its position attribute is "later". All invention allocations that are not "first" position, as well as all "additional" type symbols have the position attribute "later". All symbols listed on the classification form other than the first listed invention symbol receive the position attribute "later" .

The first step to classifying a document in CPC is to identify the subject matter that will be classified. The second step is to identify appropriate groups in CPC covering the subject matter to be classified.

The general rules for identifying the subject matter to classify in CPC are as follows:

1. (A) All subject matter covered by the claims of a patent document must be classified as invention information along with any novel and unobvious constituents or components (subcombinations) of the claimed subject matter. Classification should be based on the subject matter of each claim as a whole and on each inventive embodiment within a claim.
2. (B) Any unclaimed subject matter in the disclosure that is novel and unobvious must also be classified as invention information.
3. (C) It is desirable to classify or index any additional information complementing the invention information, mentioned in the claims or in the unclaimed disclosure, if it is useful for search purposes.
4. (D) A subcombination of the subject of the invention if the subcombination is itself novel and non-obvious.

The scheme or definitions in particular areas should be consulted to determine if additional classification or index codes are required. For example, places in CPC where multi-aspect classification is especially desirable are indicated by a note. See MPEP § 905.02. Depending on the nature of the subject matter concerned, such a note prescribes obligatory classification of the subject matter according to the indicated aspects, or contains a recommendation for multi-aspect classification if it is desirable for increasing the efficiency of the patent search.

The general procedure for identifying the appropriate groups for covering the subject matter to be classified is as follows:

1. (A) Identify the appropriate subclasses covering the subject matter to be classified.
   1. (1) The scope of a subclass is defined by its title and definition. In order to determine a candidate subclass for classification one must review the titles of available subclasses for scope coverage.
   1. (2) After a candidate subclass has been identified, the subclass notes, references and definition should be consulted to verify that the scope of the subclass covers the subject matter to be classified.
   1. (3) The preceding steps should be repeated until a candidate subclass is verified to cover the subject matter to be classified.
2. (B) Identify the appropriate group(s) covering the subject matter to be classified.
   1. (1) Identify candidate main groups that cover the subject matter to be classified.
   1. (2) Verify that the notes, references, and definitions do not exclude the subject matter to be classified, and that the group is in active use (not under reclassification).

   The technical subject matter of many inventions is completely covered by only one group in the subclass. In this situation, classification is made in the one group covering the technical subject matter of the invention.

   When multiple groups cover the technical subject matter of a single invention, classification is generally made in the group which most completely covers the invention. Within a group array, a group that covers the technical subject matter of the invention to be classified generally covers the subject matter more completely than the parent of that group.

   When multiple groups cover the invention equally well, groups are selected according to the following rules:

   1. (a) In the case where the Last Place Priority Rule (LPPR) is applicable to the groups under consideration, then the group lowest in the scheme is selected for classification.
   2. (b) In the case where the First Place Priority Rule (FPPR) is applicable to the groups under consideration, then the group highest in the scheme is selected for classification.
   3. (c) In the case where neither LPPR nor FPPR are applicable to the groups under consideration, then classification is made in each group.
   1. (3) Whenever classifying into a group, the scheme notes and definitions should be consulted for guidance regarding special rules of classification applicable to the CPC groups at issue, such as the requirement for Multiple Aspect classification or Indexing.
   1. (4) Whenever classifying into a group, the scheme should always be consulted for precedence notes and references that indicate a preference for classification in another group. Precedence notes and limiting reference should always be followed when applicable.
   1. (5) Identify application/functional classification places. Occasionally, two or more subclasses/groups are identified that cover the disclosed invention information from different aspects. For example, one classification place might cover specific uses of the invention, whereas the second classification place may cover the generic uses of the invention. These types of coverage are respectively referred to as application and functional classification places. In general, classification is made in the application classification place when application specific adaptations are disclosed. For example, a pump specially adapted to replace a heart is classified in A61M 1/10, whereas pumps, in general, without any disclosed special adaptation are classified in F04B, F04C, F04D, or F04F depending on the specific structure of the pump. When uses of inventions are nominally disclosed, classification is generally made in functional classification places.
   1. (6) Identify genus/species classification places.
      1. (a) In the case where an invention is generically disclosed classification is made to the classification place covering the generic invention. Even in cases where species are nominally disclosed, such as by name only, classification is still made to the generic classification place.
      2. (b) In the case where fully enabled species are disclosed, classification is required in the classification places covering the individual species.
   1. (7) Identify residual classification places

      In the event a classification place that explicitly covers the technical features of the invention to be classified cannot be identified, then classification should be made to a "residual" classification place. Residual classification places can be found at both subclass and group levels. Residual classification places are identifiable by their titles, which typically indicate they cover subject matter not elsewhere covered.

When an application is passed to issue, it is the duty of each primary examiner to personally review the classification picture of the application in the issuing classification boxes on the Issue Classification form. This form provides space for the full name of the "Primary Examiner" to show that the review has been made. An examiner with full signatory authority who acts personally on an application and sends it to issue should stamp and sign their name on the Issue Classification form ONLY in the "Primary Examiner" space.

In certain CPC fields, the examiner has the ability to create and search on combinations of CPC symbols (provided as groupings of symbols), each symbol in a grouping has a defined relationship to the other symbols in its grouping. These groupings are termed combination sets, and provide an enhanced mechanism for storing and retrieving classification information from patent documents. While the usage of CPC combination sets is confined mostly to the chemical fields, there are numerous instances where combination sets are used in mechanical and electrical fields.

The first symbol in a combination set is termed the base classification symbol, and determines the authorization for creation or deletion of combination sets within the field of the base symbol. The other members of a combination set possess the same Invention or Additional information, i.e. INV/ADD attribute as the base symbol, with an ordered ranking to denote their positioning within the combination set.

Guidance on the creation of combination sets is detailed in the CPC classification definitions. For example, the combination sets in a given field may denote the sequence of operations in a multi-step process, while in another field, the combination sets may denote the product and its method of manufacture. It is even possible to have the same CPC symbol appear more than once in a given combination set, with the ordering thereof to reflect the occurrence of multiple steps provided for by the repeated CPC symbol.