User enters Sales Order → goes to item screen. User enters Material + Quantity. SAP validates item → calls USEREXIT_CHECK_VBAP. Your enhancement runs. If xvbap-kwmeng < 10, show error/warning. If OK → continue. User clicks Save. SAP calls USEREXIT_SAVE_DOCUMENT_PREPARE → do final checks. SAP calls USEREXIT_SAVE_DOCUMENT → write ...