Shopify's B2B checkout includes a purchase order field, but cannot make it required or check the format buyers enter. PO Number Validator adds that control. Set your rules once, and every B2B buyer must enter a PO number matching your format before placing an order. Validation runs server-side, so it applies to every checkout type. When a PO is missing or incorrect, buyers see a clear message explaining what to fix, so orders arrive correct and ready for your accounts payable team.