Service PO Payment terms

Hi Experts,

Please guide me how to maintain the below payment terms for the service.

Service = Pest control

Payment terms will be 20 % payment on first service 30 % payment on second service and 50 % payment on third service.

In the above scenario how system will come to know to charge 20% of the total amount (Invoice booking) one first service and so on.

Also Kindly tell me what is the use of limits and functionality.

Regards,

Sri............