The shield proficiency list file is used to set the shield category a shield belongs to (e.g. Buckler, Shield (Light), Shield (Heavy), Tower Shield) so that PCGen can determine whether or not a character qualifies to use that shield or not. The 1st field is the name field, there is no tag for it, just the name of the shield proficiency.
Example:
Shield (Light)
Defines Shield (Light)
as a category of shield to be used in the associated dataset.