RDA -> Rebuildable Dripping Atomizer (you directly drip e-liquid on the wick/coil, it only holds a few drops at a time, silica wick is mainly used on these) examples are Igo-l, phoenix, nimbus, v5 etc.
RBA -> Rebuildable Atomizer (these ones have a tank that holds e-liquid and are mainly used with vertical stainless steel mesh wick) examples are Cobra, did, ZAP, AGA-T, Ody, Ithaca, RSST, etc.