49 C.F.R. § 38.4 – Miscellaneous instructions.

(a) Dimensional conventions. Dimensions that are not noted as minimum or maximum are absolute.
(b) Dimensional tolerances. All dimensions are subject to conventional engineering tolerances for material properties and field conditions, including normal anticipated wear not exceeding accepted industry-wide standards and practices.
(c) Notes. The text of these guidelines does not contain notes or footnotes. Additional information, explanations, and advisory materials are located in the Appendix.
(d) General terminology.
1. (1) Comply with means meet one or more specification of these guidelines.
2. (2) If or if * * * then denotes a specification that applies only when the conditions described are present.
3. (3) May denotes an option or alternative.
4. (4) Shall denotes a mandatory specification or requirement.
5. (5) Should denotes an advisory specification or recommendation.