41 C.F.R. § 301-10.110
If you are an employee of an agency as defined in § 301-1.1 of this chapter, you must use a contract City Pair Program fare for scheduled air passenger transportation service unless one of the limited exceptions in § 301-10.111 exists.
Note 1 to § 301-10.110: When a contract City Pair Program carrier offers a lower cost capacity-controlled coach class contract fare (_CA) and an unrestricted coach class contract fare (YCA), you must use the lower cost capacity-controlled fare when it is advantageous and meets mission needs. A listing of contract City Pair Program fares is available at https://www.gsa.gov/citypairs.
Note 2 to § 301-10.110: Employees of the Government of the District of Columbia, with the exception of the District of Columbia Courts, are not eligible to use contract City Pair Program fares even though these employees otherwise may be covered by the FTR.
41 C.F.R. §301-10.110