Help~~problems of billing split

Hi Experts,

I encountered a problem about billing split that need your kindly help~~

I tried to combine 2 sales order to 1 billing document but failed, and system split them due to "Credit data exchange rate at billing document rate", where could you see this "credit data exchange rate"?

Another question is: I created 2 credit memo and use VF01 to pull out the billing document, system split them to 2 billing doc automatically. But if I don't use VF01 to pull out it, and let system to create billing doc during the overnight billing job run, they can be combined...I can't find out the reason..

Thanks a lot~~~