Cree is the LED company, XR-E is the model, and R2 is one of the flux bins for the LED. Flux is how efficient/bright a LED is. For Cree, the more common bins, in order of efficiency/output, is P4, Q2, Q3, Q4, Q5, R2. There are others,
check here. The difference between, for example, P4-Q2 isn't big because our eyes don't sense light linearly. The lower priced companies are probably depending on this so they probably use Q5s instead of R2s because R2s are heard to get.
If you are speaking of the 4-18v drop-in, then as long as yo have enough heatsinking, it should work.