Also known as Rebuildable Dripping Atomizer. No tank to hold the ejuice, you literally drip into it. You might also see items labeled as RBA, which means Rebuildable Atomizer. RBA's usually have a tank; RDA's - you just drip, vape for a bit, and drip some more. RBA is often the most flavorful, and most often used for sub-ohm vaping. There are also items labeled RTA, which is Rebuildable Tank Atomizer that holds some juice.