CR123 is a primary battery, RCR123 stands for rechargeable CR123 and is used about any battery in CR123 that can be recharged. 16340 is a size specification and is usual used about rechargeable batteries in CR123 size, i.e. exactly the same as RCR123.
But the RCR123/16340 specification is used about a lot of different batteries:
1) LiCoO2/ICR and LiMn/IMR batteries that are 3.6/3.7 volt nominal, but 4.2 volt of the charger.
2) LiFePO4/IFR batteries that are 3.2 volt nominal (often marked 3 volt), but 3.6 volt of the charger.
3) Batteries from 1) with a voltage drop, i.e. voltage is between 0.4 and 0.7 volt lower than 1).
The size does also vary with a few mm, i.e. not all RCR123/16340 can fit instead of a CR123 cell and not all equipment will work with the higher voltage.
The Xtar WK21 uses the ones from 1) that is the same as the AW 123 batteries.