From Pathfinder Lost Omens Pathfinder Society Guide, which isn't enabled.

Armored Exercise

FEAT 14

Prerequisites Swordmaster Dedication; trained in light, medium, or heavy armor

You've spent time training in your armor alongside other agents from the Swords. Your proficiency rank for light, medium, and heavy armor increases to expert for whichever of those you already had the trained proficiency rank in.

Pathfinder Lost Omens Pathfinder Society GuideOGL