If you enter a Collection No. into this field, it will be copied to the Collection No. field of the Sales 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 Sales Header after Sales Lines have been created, the Collection No. of these existing Sales Lines will not be updated. But when you create new Sales Lines, the new Collection No. of the Sales Header will be copied to these new lines.
If the Sales Header contains an Order Type holding a Shipment Date Code, the changed Collection No. of the Sales 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.