Hope this will help .... (To define when the which way the payment has to be paid ...ex. suppose you have a invoice of 1000 and payment term is 30 % by invoice rest after 1 months so at payment of invoice first time it will take only 30 % of 1000 means 300 and again when you pay the invoice it will look of remaining due 70 % , 700 .) https://doc.openerp.com/v6.0/book/3/3_9/payment_terms.html/