A 'per diem' is something decided by your company, not by regulation. Essentially your company decides what an appropriate expense rate is per day (Latin:per diem) and agrees to pay you that, regardless of your actual expenses. This is intended to save all the hassle of collecting receipts and auditing payments, while also counteracting the tendency of people on business trips to choose the most expensive of anything they can get away with. If you get a per diem allowance you will (usually) not be reimbursed explicitly for lodging, food and similar - the per diem is intended to cover those expenses. You (usually) will be reimbursed explicitly for flight tickets or similar, if you paid for them.
Tax authorities have 'recommended' or 'maximum' per diem rates in order to guard against a tax strategy where a company pays lots of money as non-taxable expenses instead of taxable salary. Your per diem rates will be less than that, but they will not necessarily be that rate.
Per diem rates are not usually proportional to your salary, though senior employees may get slightly higher per diems.
Your company will be able to tell you what the per diem rate for the trip you are making is, and what expenses they will or will not also reimburse you for.