Service: Use this for services you can apply to your general ledger salary and wages account. This is useful for services provided by your employees. You can enter a cost for the service.
G/L Sales/Inc: Enter the default income account that will be credited when you sell service items.
G/L Salary/Wages: Enter the default salary and wages account that will be debited when you purchase service items and credited when you sell service items.
G/L Cost Sales: Enter the default cost of sales account that will be debited when you sell service items.
Note: If you are using Time & Billing (possible only in Sage 50 Premium Accounting) and you do not want the service to directly affect cost of sales, use charge (expense) items in place of service items.