Royal Mail, the UK's postal service, uses two 4-state barcodes, and Labeljoy makes both: the 4-State Customer Code (RM4SCC) with the postcode, and Mailmark, which also identifies the sender and each item.

Standard (4-State Customer Code)

Mailmark

Modes
Standard	The 4-State Customer Code: the postcode and the delivery point suffix, with no spaces - LE28HS9Z. Capital letters and digits, up to 50 characters; lowercase is turned into capitals. Singapore Post uses the same barcode.
Mailmark	22 characters (barcode C) or 26 (barcode L), as Royal Mail assigns them - see below.

What it holds
Check digit	Standard: a check character, worked out by Labeljoy and not printed. Mailmark: error correction (Reed-Solomon) built into the bars.
Readable text	The value, under or over the bars.

Writing a Mailmark. The fields follow one another with no separators:

Mailmark fields
Format	1 digit, 0 to 4.
Version ID	1 digit, 1 to 4.
Class	1 character, 0 to 9 or A to E.
Supply Chain ID	2 digits for barcode C, 6 for barcode L.
Item ID	8 digits.
Postcode and DPS	9 characters: the postcode without its space, the 2-character delivery point suffix, and spaces to fill the 9. XY11 followed by spaces marks an international item.

The rest of the settings - text position, quiet zone, color - are in the Barcode section.

What goes wrong
Invalid characters in input data.	Standard: the value has something other than letters and digits - a space, for instance.
Invalid character in input data.	Mailmark: a character other than letters, digits and spaces.
Invalid Format character in input data. Invalid Version ID character in input data. Invalid Class ID character in input data. Invalid Supply Chain ID character in input data. Invalid Item ID character in input data.	Mailmark: that field has a value it can't take - see the table above.
Invalid Postcode in input data.	Mailmark: the postcode and DPS field isn't a UK postcode followed by a DPS - or the value is too short to reach it.
Input data too long.	More than 50 characters (Standard) or 26 (Mailmark).