It's where the transit relay plugs in to avoid draining of the battery during transportation from the factory to dealer, and if left in storage before being handed over to the end user
"The purpose of the Transit Relay is to prevent the battery from being discharged during transit and Dealer storage. It automatically isolates the battery following ignition off. If the vehicle is to be handed over to a customer within a period of 24 hours of completion of the PDI, the Transit Relay MUST be removed. If the vehicle is NOT to be handed over to a customer within a period of 24 hours of completion of the PDI, a Transit Relay Test Lead must be fitted to allow the PDI to proceed. Under no circumstances should any means other than the test lead be used to bridge the Transit Relay. The Transit Relay Test Lead MUST be removed after the completion of the PDI and stored for further use. Under no circumstances should the vehicle be transported with the Transit Relay Test Lead fitted. The procedure to fit the Transit Relay Test Lead and remove the Transit Relay is given in the PDI Manual"