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

Signifer Armor Expertise

FEAT 12

Prerequisites Hellknight Signifer Dedication

You've spent enough time helping your comrades equip medium and heavy armor that you spread your own expertise to those armors as well. If you have expert proficiency in any armor or unarmored defense, you also gain expert proficiency in medium and heavy armor.

Pathfinder Lost Omens Character GuideOGL