get individual clearing doc no. for every document number at settlement run

hi

i am using tcode FCC1 for settlement run, after the document' settlement is done i see that the clearing document number is same for every document number,

my requirement is i want individual clearing document number for each document number,

I debugged the program RFCCSSTT and found out that this posting is done through a standard FM, so i also checked the FM , but was unable to find out any place where this clearing document field is called or even mentioned.

please help as to how to achieve this .

thanks

Swarn