Explanation/Reference:
Sold-to BU: If the selected BU performs invoicing, then the same BU can be used as the Sold-to BU.
However, in certain instances due to tax or legal reasons the client BU may only perform requisitioning with the supplier and another BU assumes the liability for the purchases. In this case a different sold-to BU is selected. The list of soldto BUs that can be selected for a given client BU is determined by the Intercompany Transaction Flow setup. In addition, when a separate Sold-to BU acts as an agent to the client BU an assignment must be created for the soldto BU in order for the BU to process invoices.
References: https://docs.oracle.com/cd/E56614_01/procurementop_gs/OAPRC/F1007476AN1079E.htm