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Tick box at the bottom left hand side of the screen.
Tick if you want this consignment to be consolidated to a previous consignment (based on Receiver details, Carrier Service)
Note Dependent on whether the Carrier Service will allow consolidation. (Priority services typically only allow 1 item per Consignment)
Un-tick if you don't want the new consignment to consolidate to an existing consignment or have it consolidate to any future consignment.