Calculation of settlement periods

Hi,

IN CRMD_ORDER I select a Sales Order. I go to an item and then i see the the tab BillPlan.

There i change the settlement Rule say from Monthly to Weekly.

The Settlement periods in the table below get populated automatically.

Can anybody tell me the function module or method where the data is calculated.

I have debugged Function Module CRM_ORDER_MAINTAIN. But i am not able to identify where the settlement periods change?

I am assuming that this table is the data i see in table IT_BILLING when i execute CRM_ORDER_READ .