Special Delivery Guaranteed by 1pm
Delivered by Royal Mail
Postage on Account GB
I have no idea what the one in black means but the white ones are for sorting and directing the parcels from the hub to their nearest local mail centre. In terms of the letters is (roughly) A** - Scotland (- Dumfries) L** - West Midlands, Norfolk, Cambridgeshire, Jersey, Guernsey, Isle of Man E** - North-east, Yorkshire, C** - North-west, North Wales, Northern Ireland, Dumfries + Carlisle X** - South-west coast, South Wales, Bristol R** - Home Counties, Suffolk, Q** - BFPO, London, Brighton A few others for International and non-geographic postcodes e.g. Amazon returns (XX10). There are about 50 bays, most of which serve more than one letter-number combination each, so I imagine on reaching the local mail centres they are further sub-sorted into their individual codes. Most of the sorting is done automatically but bulky stuff and stuff which for other reasons won't go on the conveyer/through the scanner properly there will be an area for manual sorting.
The black boxes are used for sorting mail within the areas designated by the white boxes.