You can turn ‘P.O. required’ flag on in Customer Center to make PO field in Service Order mandatory, but this will also make PO mandatory in Customer Contract.
This will create an issue when a customer provides PO only at the service order level. When PO is mandatory in Customer Contract, PO field in a new service order gets a default PO value from a contract, so the system will not force the user to enter a PO value unique to the service order.
Is there any way to make PO field mandatory only in Service Order, but not in Customer Contract?