At one point in time only manually inflated jackets were approved, as a type 3. Then automatically inflated jackets were approved as a type 5-the reason being that it was felt that automatics, having more intricate inflation mechanisms, were more complicated and therefore should be worn to be legal. (and yes, automatics can also be manually inflated, before it hits the water.) That would have been an easy rule to remember-manuals are type 3, autos are type 5 and have to be worn. but then type 2 autos came along, and new jacket designs came along, and then the rule just kind of went away. Now you have no real way of knowing what a jacket is unless you look at the label.