This documentation covers the details of Using the LockBox Files in SAP FICO General Ledger
Lockbox is a process provided by a bank where the customer remits his payment to a PO Box at your bank, the bank deposits the check in your account and enters the remittance data from the customer. Then the bank
sends the data to you electronically for you to import and apply in SAP-AR.
The structure that the bank uses is usually one of two formats. BAI will provide you the customer information and the check amount but no invoice remittance data. Usually cheaper and works well for a business where customers are paying only one invoice at a time.
BAI-2 offers remittance data but the bank charges more for the service because they have to enter more info.
Look at structures FLB** to see the transfer data.
When the bank sends the customer payment data you use Treasury function FLB2 to import and then process the payments. This process attempts to determine where to apply the payment and will post directly against an invoice, an account to a customer or if it can’t even determine the customer it posts into a clearing account (configured) to be resolved.