Adding custom fields to financial report FBL5N using Business Transaction Events (BTEs). The document describes how to add Billing Plant and Description columns to FBL5N by creating a BTE product, assigning it to the appropriate FBL5N execution event, and writing a function module to extend the RFPOS and RFPOSX structures to include the new fields. The function module is then called to populate the additional columns when FBL5N is run.