Reimbursement means that a taxpayer's employer covered an expense that was originally paid for by the taxpayer.  Reimbursed expenses are not tax deductible.  Per diem is a form of reimbursement.  An employer that pays per diem via an accountable plan is reimbursing the crewmember for their meals and incidental expenses while on work-related travel.

The reason the per diem deduction generates an increased tax refund for flight crewmembers is because the per diem from an employer almost never exceeds the crewmember's M&IE expenses when using the city-by-city CONUS and OCONUS per diem rates that can be used in lieu of actual meals costs incurred by a flight crewmember.  In other words, because the reimbursement (per diem from the employer) is less that the expense (M&IE per diem deduction), a tax write off for meals and incidental expenses is allowed.

These can simply be calculated via the EZPERDIEM.COM Per Diem Calculator ^TM.