grouping of invoices using invoice list

I have advised my client to use SD>Billing>Invoice List>Maintain output for invoice list to achive grouping of invoices

i noticed under

SD>Billing>Assemblin groups

one is assembling group of billing documents

and other one is Assembling group of invoice list

Please tell me only the difference of both and where and through which t code to see the ouput of assigned billing doucment

now after configuring SD>Billing>Invoice List>Maintain output for invoice list i see ZPGR for invoice with F group type coming in VG02, should use VG02 to manually assign billing document..

Regards

Siva