The underlying system behind this is Royal Mails augmentation of the UPRN system with the UDPRN (Unique delivery point reference number) which is meant to be 1:

The underlying system behind this is Royal Mails augmentation of the UPRN system with the UDPRN (Unique delivery point reference number) which is meant to be 1:1. There is then a further classification for multitenant units called the UMPRN (unique multiple residence number)