RBA: ReBuildable Atomizer (all rebuildable toppers, including tanks and drippers).
RDA: Rebuildable Dripping Atomizer (no tank, eg Derringer)
RTA: Rebuildable Tank Atomizer (with a tank, eg Kayfun)
RT/DA: Rebuildable Tank/Dripping Atomizer (RDA with a juice supply, eg Big Dripper)