External Code List are accepted values that sit in another file in the same location as your rules file.
If we wanted to validate zip codes, countries, CPT codes, or ICD codes, for example, we could also add them using the same method. The issue, however, is that these lists are much larger. This is where external code lists enter the picture. They allow an element’s accepted values to be located in an external file.
The first step is to create an external code list. Let’s create a file called States.txt and add a few accepted values to it. Only the code is required, the description is optional