The KIX code - KlantIndex - is the 4-state barcode of PostNL, the Dutch postal service. It carries the postcode and the house number, so mail can be sorted by machine.

What it holds
Characters	Capital letters and digits, with no spaces: the postcode, the house number and, if there is one, an X and the house number's suffix - 1231FZ13XHS is postcode 1231 FZ, number 13, suffix HS. Lowercase is turned into capitals.
Length	Up to 18 characters.
Check digit	None.
Readable text	The value, under or over the bars.

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

What goes wrong
Invalid characters in input data.	The value has something other than letters and digits - a space, for instance.
Input data too long.	More than 18 characters.