Per diem is reimbursement primarily for employees on official travel, although it is paid for relocation, as well.
It consists of two parts. The lodgings portion varies among some 400 localities whose hotel and motel costs are studied annually; for other areas, a flat rate applies. You must submit your actual cost of lodging for reimbursement, supported by receipts.
The second portion is the meals and incidental expenses portion. It also varies among the designated localities, with a flat rate elsewhere. That portion is payable without itemization of expenses or receipts.
In limited circumstances an agency may reimburse for actual expenses higher than the allowable per diem rates.
Rates are set on a fiscal year basis and some may change during the year. Separate rates apply to foreign travel and rates for non-foreign travel outside the contiguous states.
Policies applying to Defense Department employees are less generous in certain ways, in particular the lodging reimbursement rates for those who are on extended temporary duty assignments.
See Chapter 11, Section 4 of the Federal Employees Almanac.