What even is armor proficiency?

[Goobahfish]

Yeah, but I've read the descriptions of military training for multiple nations...and nowhere is training to be used to armor mentioned.

There is training and getting used to carrying heavy loads, marching long distances, sprinting while carrying like 30-50 pounds on you, ect ect.

Granted, plate armor is different, but that's plate armor. I don't think the training to carry a mail shirt is that much different than training to hike while carrying approximately the same weight.

This.

Training to use armour... it using armour for a day or two until you adjust. Armour proficiency should mostly be tied to stats rather than class. 'Training' in armour is really a bit of a misnomer.