49 C.F.R. § 821.3
In addition to sequential numbering of cases as received, each case formally handled by the Board will receive a letter prefix. These letter prefixes reflect the case type: "SE" for safety enforcement (certificate suspension/revocation) cases; "SM" (safety medical) for cases involving denials of medical certification; "CD" for cases involving non-medical certificate denials; "SR" for cases involving safety registration issues under 49 U.S.C. 44101 et seq.; "CP" for cases involving the imposition of civil penalties; "NA" for cases in which a petition for review or appeal is not accepted because of a patent procedural deficiency; and "EAJA" for cases involving applications for fees and expenses under the Equal Access to Justice Act, governed by Part 826.
49 C.F.R. §821.3