It's a tank, more specifically a stock coil tank.
RTA stands for Rebuildable Tank Atomiser, the key word being "rebuildable".
If a tank takes stock coils, it's not an RTA. I suppose it might be a TA, at a push.
Some "TAs" have an option of an RBA (ReBuildable Atomiser) head, which replaces the stock coil - but it doesn't make it into an RTA.
(RDA is Rebuildable Dripping Atomiser, dripping meaning you drip on the liquid instead of having it fed from a tank.
Then you have RSA/RDSA, RDTA and I'm also proposing RDSTA.)