41 C.F.R. § 301-11.2

The agency must pay subsistence expenses (either a per diem allowance or actual expense) unless:

(a) The travel is to a training event under the Government Employees Training Act (5 U.S.C. 4101-4121), and the employee agrees not to be paid subsistence expenses; or

(b) The travel is for a pre-employment interview, and the interviewing agency does not authorize subsistence expense payment.