The "T" is actually a small cross piece in the atty that the cartridge or tank is punctured by. You can use a non-T cartridge on a T atty, but not a T cartridge on a non-T atty. If the hole created by the pucture gets too big, or if you twist the tank and the T pops the cartridge/tank cap off, you can have a big leaking issue.
You don't use an atty with a cartomizer (its built in). Think of the atty as your lighter.