10 U.S.C. § 1591

1. (a)Subject to subsection (b), the Secretary concerned may authorize reimbursement to a civilian employee who is accompanying a Member of Congress or a congressional employee on official travel for actual travel and transportation expenses incurred for such travel.
2. (b)The allowance provided in subsection (a) may be paid—
   1. (1)at a rate that does not exceed the rate approved for official congressional travel; and
   2. (2)only when the travel of the member is directed or approved by the Secretary concerned.
3. (c)In this section:
   1. (1)The term “Member of Congress” means a member of the Senate or the House of Representatives, a Delegate to the House of Representatives, and the Resident Commissioner from Puerto Rico.
   2. (2)The term “congressional employee” means an employee of a Member of Congress or an employee of Congress.
   3. (3)The term “Secretary concerned” includes the Secretary of Defense with respect to civilian employees of the Department of Defense other than a military department.