The different bar code types are intended for different applications, i.e., the Postnet is the one used by the U.S. Postal Service and it contains information (including the ZIP code) that allows them to sort letters mechanically.
The most widely used bar code is Code 39. Because it contains numbers, letters and some characters such as dash, dollar, slash, etc., it can be used with existing numbering systems.
The UPC (Universal Product Code) is the main system used by retail stores, but it contains ONLY numbers, and must have a fixed number of characters in it.
The Interleaved 2 of 5 is the system used for industrial purposes, and is the standard for shipping containers. The main advantage of this system is that it is very compact, thus making it suitable for use where space is limited.
Code 128 is the only code that has all 128 ASCII characters so the bar code can contain alpha, numeric, punctuation or special characters.
Printing bar codes may present a issue if you use an ink jet printer. The issue may arise if the bar code reader is intended to read "invisible light". There are two types of readers: visible light and invisible light readers. With invisible light readers, the reader "needs" a carbon content in the ink, which ink jet ink does not have. This means that the barcode will print nicely, but the reader will not read it. If you print something on an ink jet printer and the bar code reader cannot read it, try making a "Xerox" copy of the printed page and see if it reads, if it does, then you know that the issue is the printer ink.
A very important setting in ClarisWorks when using bar codes is the "Fractional Character Widths" in Text Preferences.
Bar codes cannot be compressed or reduced. This means you have to use the correct size font. This is why when you purchase a commercial bar code font it comes in various sizes.