Fixed Value Date - Billing Document

Hi,

     I have a combined billing Idoc which carries around 17 SO. But the Invoice get split in to two, once has 16 SO and another one has 1 SO. When I checked the header data of both the billing documents, I found FIXED VALUE DATE is empty in one SO and another one has value in it which is same as billing date. Please tell me the logic behind this Fixed Value date. As I am a beginner and ABAPer, please explain me why it gets empty and also if possible please let me know if there is any standard function module or program which calculates this Fixed value date for invoice.

Thanks

Hariny S