41 C.F.R. § 301-11.1 : Public Contracts and Property Management — FEDERAL TRAVEL REGULATION SYSTEM — TEMPORARY DUTY (TDY) TRAVEL ALLOWANCES — ALLOWABLE TRAVEL EXPENSES —11 - PER DIEM EXPENSES — GENERAL RULES —11.1 - When am I eligible for an allowance (per diem or actual expense)?

When:

(a) You perform official travel away from your official station, or other areas defined by your agency;
(b) You incur per diem expenses while performing official travel; and
(c) You are in a travel status for more than 12 hours.

41 C.F.R. §301-11.1