The Document Number (Doc. No.) field on Shipping Label stands for the PO# from the sales order form.
Follow Below Steps:
1. Navigate to Transactions > Sales > Enter Sales Order.
2. Fill in the form.
3. Enter the PO#{ otherrefnum} value.
4. Click Save and Fulfill the order.
5. Click Save on the item fulfillment form.
6. From the item fulfillment record > click the Print > Print Label and Doc. No. element shows the PO# field value as below on the Shipping Label
