Per diem rates refer to the rates set forth by the GSA (CONUS) or Department of State (OCONUS). Every location in the world has a per diem rate attached to it, and each per diem rate for each location changes with time. Both DoS and GSA Per diem rates are important to flight crews because the IRS allows taxpayers to use the per diem rates as an alternate method of accounting for their meals and incidentals expenses (M&IE) on the employee business expense portion of their taxes. Basically, the per diem rates are looked up using the location of each layover that a flight crewmember stays at throughout the year. That rate is then adjusted depending on whether or not the layover occurred in the middle or end of a trip. Then all of those adjusted per diem rates for the tax year are added together to give the taxpayer their M&IE calculation which is used to determine their per diem deduction. All of this is done automatically by EZPerDiem.com when a pilot or flight attendant enters their layovers for the year into the EZPerDiem Per Diem Calculator ^TM.