Is there any possiblity to make the PO number as non mandatory grouping attribute

Summary:

The client requirement is to group the order lines based on the INTERFACE_LINE_ATTRIBUTE7 (packing slip id). So we created the Custom group rule called DELIVERY GROUPING to group the order lines based on packing slip id. Due to PO number is mandatory grouping attribute, system considering the PO number first for grouping, then its considering the custom grouping rule for grouping the order lines.

Client requirement is to group the order lines based only on packing slip number. Due to system standard behavior, order lines are grouping into invoices based on PO number first then packing slip number. Please refer the attachment to understand how system is grouping the order lines into invoices.