Field help (BPSNUMSHIP) 

Help common to the Shipments, Shipment pre-receipt and Order pre-receipt functions.
Use this field to specify the supplier to which the purchase orders or the freight containers that you want to select are linked. Choosing a supplier allows you to filter in the selection panel only those purchase orders placed with this supplier or the freight containers associated with this supplier.
If you have also entered a site:

Once the first order or the first container has been selected, the supplier can no longer be modified.
For a given shipment, if you want to save orders placed with different suppliers, or freight containers defined for different suppliers, leave that field empty and use the Picking criteria to carry out your selection.