User blog comment:IamJakuhoRaikoben/Chapter 456: Order, Review./@comment-25085824-20151012144241/@comment-7418318-20151012180601

The summoning of the weapons and armour itself is caster, but the magic she uses in those armours is holder, since the power comes from the items.

For example we've never seen her use fire magic, but in Flame Empress she can shoot flames. Same with Lightning and Water in the other two Empress armours or how she can fly with Black Wings and Heaven's Wheel armours.