If you enter a Collection No. into this field, it will be copied to the Collection No. field of the Purchase Line when you select a Master No. in the No. field.
This field can also be filled in automatically if you enter an Order Type containing a Collection No..
If you change the Collection No. of the Purchase Header after Purchase Lines have been created, the Collection No. of the existing Purchase Lines will not be updated. But when you create new Purchase Lines, the new Collection No. of the Purchase Header will be copied to these new lines.
If the Purchase Header contains an Order Type holding a Shipment Date Code, the changed Collection No. of the Purchase Header will take over and interact with the Shipment Date Code of the Order Type, depending on the setting of the Shipment Date Code Handling.