Expenses/reimbursement.
If they pay you a set amount (an expense "allowance") without making you account for how you spend it, then they are supposed to include it as taxable income on your W-2, in box 1. For tax purposes, that income is treated as regular wages and is taxable to you.
If, on the other hand, they have an "accountable plan", wherein they only reimburse you for your actual expenses, then they do not include it on your W-2. It is not taxable to you.
