49 CFR 33.31 - Priority ratings.
(a) Levels of priority.
(1) There are two levels of priority established by the Transportation Priorities and Allocations System regulations, identified by the rating symbols “DO” and “DX”.
(2) All DO-rated orders have equal priority with each other and take precedence over unrated orders. All DX-rated orders have equal priority with each other and take precedence over DO-rated orders and unrated orders. (For resolution of conflicts among rated orders of equal priority, see § 33.34(c).)
(3) In addition, a Directive regarding priority treatment for a given item issued by the resource agency with priorities jurisdiction for that item takes precedence over any DX-rated order, DO-rated order, or unrated order, as stipulated in the Directive. (For a full discussion of Directives, see § 33.62.)
(b) Program identification symbols. Program identification symbols indicate which approved program is being supported by a rated order. DOT will use the letter “T” followed by a number for all transportation-related approved programs. Programs may be approved under the procedures of Executive Order 13603 at any time. 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 DO-T1 or DX-T1 for a priority rating under TPAS.