MPEP § fp18.18
As provided in 37 CFR 1.475( a), a national stage application shall relate to one invention only or to a group of inventions so linked as to form a single general inventive concept ("requirement of unity of invention"). Where a group of inventions is claimed in a national stage application, the requirement of unity of invention shall be fulfilled only when there is a technical relationship among those inventions involving one or more of the same or corresponding special technical features. The expression "special technical features" shall mean those technical features that define a contribution which each of the claimed inventions, considered as a whole, makes over the prior art.
The determination whether a group of inventions is so linked as to form a single general inventive concept shall be made without regard to whether the inventions are claimed in separate claims or as alternatives within a single claim. See 37 CFR 1.475(e).
As provided in 37 CFR 1.475(b), a national stage application containing claims to different categories of invention will be considered to have unity of invention if the claims are drawn only to one of the following combinations of categories:
Otherwise, unity of invention might not be present. See 37 CFR 1.475(c).
1. Begin all Lack of Unity actions in national stage applications submitted under 35 U.S.C. 371 (including species) with this heading.
2. Follow with form paragraph 18.19 or 18.20, as appropriate.
3. For lack of unity during the international phase, use form paragraph 18.05 instead of this form paragraph.