The pay code window contains a table of information (pay codes) that control how an account behaves when a paycheck is processed.
The normal balance of the account determines whether the account is an earning or a deduction. Debit accounts are considered an earning because they increase the net amount of the paycheck, whereas credit normal balance accounts reduce the net amount, and are therefore considered deductions.
The pay codes for earnings and deductions accounts are quite similar. Both contain controls over the amount of the earning or deduction, and the influence on income tax and other calculations.
See also Pay Code Earnings Window and Pay Code Deductions Window