BPOs are a type of purchase order used for long-term purchasing agreements between an agency and a vendor. Goods or services are procured during this time period (called service start and end dates) without the agency having to create individual purchase orders for each purchase.
The invoices for the BPO must be within the BPOs service start and end dates, although the agreement can include a grace period, during which time the vendor can continue to send invoices.
For more information. review the information in the topic: