When forwarding the billing documents to financial accounting, posting was generally carried out with billing type 'RV'. This meant that evaluations could not be carried out using the document type, i.e. listing all credit memos that had been forwarded using the interface from FI to SD. It also made it impossible to differetiate between document types in the number ranges.
By maintaining the billing types in Customizing, you can differentiate between the document types.
If no entry is made fro the billing type, then posting is still carried out using billing type 'RV'.
You maintain the billing type settings in the the 'document type' field. "Maintain billing type".