A Barrier Jacket is summoned when the mage activates their Device (mages who don't have a Device summon it manually) and is accompanied by a bright flash of light. Whatever normal clothes the mage is wearing at the time disappear until the Barrier Jacket is dismissed. It has been shown on at least one occasion that it is possible to activate a Device without summoning the Barrier Jacket, however, any situation requiring the former is also most likely to warrant the latter, anyway.