15 CFR 700.11: Priority ratings.

(a) Levels of priority. (1) There are two levels of priority established by this regulation, identified by the rating symbols “DO” and “DX.”

(2) All DO rated orders have equal priority with each other and take preference over unrated orders. All DX rated orders have equal priority with each other and take preference over DO rated orders and unrated orders. (For resolution of conflicts among rated orders of equal priority, see § 700.14(c).)

(3) In addition, a Directive issued by Commerce takes preference over any DX rated order, DO rated order, or unrated order, as stipulated in the Directive. (For a full discussion of Directives, see § 700.62.)

(b) Program identification symbols. Program identification symbols indicate which approved program is being supported by a rated order. The list of approved programs and their identification symbols is found in schedule I to this part. For example, A1 identifies defense aircraft programs and A7 signifies defense electronic and communications equipment programs. Program identification symbols, in themselves, do not connote any priority.

(c) Priority ratings. A priority rating consists of the rating symbol—DO and DX—and the program identification symbol, such as A1, A7, or N1. Thus, a contract for the production of an aircraft will contain a DO-A1 or DX-A1 priority rating. A contract for a radar set will contain a DO-A7 or DX-A7 priority rating.