The armor proficiency list file is used to set the armor category a piece of armor belongs to (e.g. Padded, Leather, Studded Leather, Chain Shirt, etc.) so that PCGen can determine whether or not a character qualifies to use that armor or not. The 1st field is the name field, there is no tag for it, just the name of the armor proficiency.
Example:
Studded Leather
Defines Studded Leather
as a category of armor to be used in the associated dataset.