Prerequisite: Armor proficiency (light).
Required for: Armor proficiency (heavy).
Specifics: This feat grants the knowledge to make effective use of medium armor. Characters without this feat cannot equip medium armor.
Use: Automatic. Fighters, barbarians, paladins, clerics, druids, favored souls, arcane archers, blackguards, dwarven defenders, and warpriests automatically have armor proficiency (medium) as a bonus feat. They need not select it.
DnD 3.5 comparison
Characters could wear armor without the appropriate proficiency, but suffered penalties to all skills involving movement and attack rolls.
- NWNWiki:Armor Proficiency (Medium)