Per diem is an allowance for lodging, meals, and incidental expenses. The US General Services Administration (GSA) establishes the per diem reimbursement rates.By including a per diem amount in the letter, the parties will not have to execute another payoff letter if the termination date. If you are on a base or post where the dfac is not available to you, then you get the full per diem regardless of whether you are in base or post lodging. This relates to the per diem rate. Bales asked if the reductions for per diem rates for regional prisons is within the intent of statute.