Trying to wrap this all into a simple response...
The new "sub-tanks" are the latest generation of what used to be called clearomizers - tanks with replaceable coils that came built and wicked from the factory that you can unscrew and throw away when they're roached, replacing with a new store-bought unit or, for the intrepid, tear apart and rebuild at home. Some of them, namely the Kanger Subtank, come with screw-in units that are specifically made to be rebuilt at home. Most use coils with a resistance below 1 ohm, hence the "sub-ohm" or "sub-tank."
RBAs are "ReBuildable Atomizers" with tanks, as opposed to RDAs, or "Rebuildable Dripping Atomizers" without.You could build them to run sub-ohm, or at higher resistances, but the key is that you build them yourself, there's no option to buy a plug-and-play solution from the tank's manufacturer.
If you're using a sub-tank with the rebuildable attachment, technically you've got an RBA, though most would still consider it a sub-tank because it has a factory coil option.