Field help (FCYSHIP) 

Help common to the Shipments, Shipment pre-receipt and Order pre-receipt functions.
This field is used to specify the site to which the purchase orders or the freight containers that you want to select are linked. Choosing a site allows you to filter in the selection panel only the purchase orders and freight containers recorded for this site. Once the first order or the first container has been selected, the site can no longer be modified.
In a shipment, if you want to save orders or freight containers originating from different sites (but belonging to the same company), leave that field empty and use the Picking criteria to carry out your selection.