Thanks a tonne Naresh.
The FM BUPA_INBOUND_MAIN_CENTRAL exists both in CRM & ECC. There inside BUPA_CONV_EI2BAPI there's a Subroutine PERORM execute_central BP is created at Line 73.
Business Object BUS1006 is called.
But still didn't find the place where KUNNR is populated.
For your second point IS_CUSTOMER = 'X' in BP classification, where do I find this ?
Regards,
Rohit