Field help (PPPDATMAX) 

Open items from: (1). to (2).
If a date range is entered, only the open items whose due dates are contained within this range will be taken into account for the payment generation.

Early discount until: (3).
If a date is entered, all the open items without early discount code and the open items with an early discount code lower than the entered date will be taken into account for the payment generation.

In a nutshell, if all the dates are entered, the following open items will be taken into account: