You need to define the payment terms in OBB8 and there you need to select the check box" instalment payment"
and here you need to give the percentage and no of days, orelse you can maintain in OBB9.
Then you need to assign the payment terms in Vendor master or Vendor invoice, so that the system will calculate the instalments based on payment terms and it will get posted accordingly.
Hope understood and let me know if you have any concern still.
the purpose of Payment terms is to allow customer to make payments in installment basis for a business transaction i.e you can split payable amounts into installments and assign different payment terms.
in obb8 you select installment check box, and in obb9 you specify installment terms and respective payment terms.
ex. first installment 30% with payment term1
second installment remaining 70% with payment term2.