The Approver user with catalog SAP_MM_BC_PO_WIAPPROVE_PC is allowed to Edit Purchase Orders
SAP S/4 HANA Cloud
The front end user that presses the “approve” or “reject” button and thus happens to establish the final status of the approval workflow work item.
Workflow engine calls have to set the purchase order status as “approved” or “rejected” that means with the same Authorizations that the front end user has.
Thus in effect, If the front end user does not have the change authorization on PO , then the Approval process shall fail.
SAP_MM_BC_PO_WIAPPROVE_PC (Materials Management - Purchase Order Approval) is assigned to the Approver user role. The catalog brings the authorization to change purchase orders because this is required for the approval of a purchase order.
KBA , MM-FIO-PUR-PO-WFL , Purchase Order Workflow , MM-FIO-PUR-PO , Fiori UI for Purchase Orders , How To