A number of common types of payments are already defined in the system. You can use the predefined payments, and add more of your own. See Common Pay Items for more details. There is no limit on the number of different payment elements.
Once you have entered a particular payment in the Calculation screen in a tax year, you will not be able to modify the accumulator definition in the payment set up screen. If you do so the change will not be saved. The only way to change an accumulator is to create a new payment with the setup you want and use an adjustment payslip for each employee to debit the old payment and credit the new.
When you select a pay item for edit, or add a new pay item, the Pay Item Edit form is displayed.