The United Kingdom has implemented a 4-state barcode referred to as the Royal Mail 4-State Customer Code (RM4SCC).
 
An example:

 

This barcode type is used by the Royal Post. It encodes 5, 6, or 7 character post codes along with a two character Delivery Point Suffix (DPS). A checksum character is included to verify the barcode read.

By default, Barcode Xpress only recognizes Royal Mail barcodes adhering to the RM4SCC specification. However, we have introduced the RoyalMailVariableLengthDecoding property to allow the recognition of Royal Mail barcodes of any length. This property is set to false by default.