What is the difference between taxable per diem and nontaxable per diem?

Per diem reimbursed to a crewmember from their company can be either taxable per diem or nontaxable per diem.  Here is how they are different:

Taxable per diem

• Shows as part of salary in Box 1 of a W-2
• Is not considered a reimbursement by the IRS.  It is a wage.
• Is paid during day-trips when a trip does not have a legit rest period (i.e. layover)
• Does not affect a per diem calculation.
• Does not get entered on IRS Form 2106.

Non-taxable per diem

• Shows in one of the following places: (W-2 Box 12 with Code L, W-2 Box 14 (probably says nontaxable per diem next to it), or last paycheck stub of the tax year)
• It is a reimbursement
• It should be less than the meal allowance (per diem calculation)
• Accumulates on trips with layovers (rest periods)
• Offsets the per diem calculation
• Gets entered on IRS Form 2106