When an employer reports an employee's earning at the end of the year on a W-2, per-diem is listed separate from taxable income, under 'Misc. non-taxable'.
Per diem is designed to cover the additional expenses you have of
There are no ramifications if you later take a full time position
They can continue to pay the per diem as long as you're working for
Any tour of duty adding up to over 500 miles counts as a per diem.