Generally, reimbursements under an accountable plan are not taxed and are not included on your W-2. With an accountable plan, you are reimbursed for qualified expenses that you provide a receipt for.
With an unaccountable plan, under most circumstances every penny you are paid is taxable income regardless of what it was paid to you for. Are there exceptions? Yes. But without knowing the specifics of your business and why you are being temporarily assigned, the possibilities are to many to cover here.
