A Bill to Account transaction can be configured to capture a reference number as a detail of the tender type.
This reference number gets appended to the description of the adjustment that is created.
For example, if the adjustment description is set to BTA for transaction 1234 (the receipt number), and a reference number of 5678 is entered, the number 5678 gets added after the transaction number. So the associated invoice would display BTA transaction 1234, Ref # 4567. This reference number might be a purchase order number, depending on your company's business rules.
Configuration is done on the Register Policy window (accessed via Applications > POS Back Office > Locations) by selecting the Prompt for BTA Reference Number check box.

The prompt for Reference Number, if configured, is displayed at the time of tendering.

Entering a Reference Number is optional; you can leave the field blank and click OK to continue.