invoice field in BSID

Hi all,

My understanding for clearing invoice created in SD is that when the invoice are created they also hit BSID-XLBNR field. When we recieve payment then lockbox will clear open invoices based on this BSID-XLBNR field.

However somebody says that BSID is secondary index & i can not clear invoices through BSID. But I will need FI core table BSEG for this. He also said we dont get any info. through XLBNR.Can some body explain me the logic behind this invoice clearing.Because I dont know if I am missing some thing.

Thanks

Nik