This is a map of relationship in base line date, posting date and net due date in payment program.
If your net due date is after posting date, value date, which is often used to send to bank, will be the net due date. Otherwise, if net due date is before posting date, value date will be the posting date. Another note, if today is value date, the bank will count from tomorrow. It mean that in acknowledgement file from bank, the value date will be determined as tomorrow.
If you carry out a partial, residual clear items of which, baseline dates are different. Each line items of new document created will be get from relevant items. For e.g, your vendor have 2 open items, one dr 300 baseline 20.08, one cr 400 baseline 20.07. You run F-44 to clear those items. New document will have two lines, One line will base on 20.08, other will based on 20.07.