The per diem deduction is misnomer or false name for deducting M&IE expenses.  Because per diem rates are used as the basis for calculating your M&IE expenses, the name "per diem deduction" stuck.  M&IE expenses stands for meals and incidentals expenses.  Therefore, the per diem deduction is simply the tax deduction that flight crewmembers are eligible for to write-off M&IE expenses when they travel for their employment.