Approval of timesheets can be done by either the approver as set up on the resource card, or the job responsible. When a user is unavailable (out of office) to approve the time sheets, a time sheet admin can approve the time sheets. But that admin is usually a specific user, that you don't necessarily want approving all timesheets!

What is missing is a substitute for each user:
Then when a user is unavailable, their substitute will be able to approve the timesheets the first user is responsible for, but only these and no other timesheets.