ProtoMechs are 'Mechs. Elementals, Battlesuits and the MechAssault Battle Armor are powered armor.
The distinction is more common sense than anything else. If it is a suit that you wear, that has some capability to propel itself, it's powered armor. If you operate it, it's a giant robot or mecha or whatever. That's like confusing an exoskeleton with an Abram's tank. You don't wear a tank, you drive it.