Codabar - also known as Code 2 of 7, NW-7 or USD-4 - is an old, sturdy barcode that prints well even on simple printers. Libraries, blood banks and photo labs still use it.

What it holds
Characters	Digits and - $ : / . +, between a start letter and a stop letter: A, B, C or D.
Length	At least three characters: the start letter, something to encode and the stop letter.
Check digit	None.
Readable text	The value without its start and stop letters.

Start and stop letters. You type them, at the ends of the value: A40156B encodes 40156 and starts with A and stops with B. Any of A to D works at either end - your scanner or the system that reads the codes may expect particular ones. Lowercase is fine.

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

What goes wrong
Invalid start character in input data.	The value doesn't begin with A, B, C or D.
Invalid stop character in input data.	The value doesn't end with A, B, C or D.
Invalid characters in input data.	The value has a character Codabar can't hold, or a letter from A to D in the middle.
Invalid number of characters in the input data.	There's nothing between the start and stop letters.