"Per Diem", as it relates to how trucking companies pay their drivers, is a portion of their salary paid to the driver un-taxed, technically as a meal and expense reimbursement.
Company drivers used to be able to deduct their per diem. As of the beginning of 2018 that has gone away. Some drivers want their company to continue to pay them a per diem, and some companies are doing so, but not all. Why do some companies hesitate to offer you a per diem?