The Delivery Point Suffix (DPS) is a code assigned by Royal Mail and used with a postcode to create the machine-readable RM4SCC barcode. The DPS identifies a specific delivery point at a property. It consists of one number (1–9) followed by one letter (A–T). 

If a DPS has not been assigned, Royal Mail uses a default of 9U or 9Z. The value 1A is reserved for postcodes marked as Large User (PostCodeType = L). 
 

Example: 

16 St Alphonsus Road, London SW4 7AS 

DPS: 1B