The Position on the Employee Maintenance card is simply the default position that is populated in Payroll Transaction Entry. The Position can be changed for each entry in the payroll transaction to change the reporting as you suggest. The Payroll Posting Account Setup will need the correct combination of Department, Position, and Pay Code to control posting to the proper GL accounts. "ALL" can be used in any column when appropriate but does not have to be used, as long as all combinations of Department, Position, and Pay Code are included.
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