its the emitter bin. (LED) all are XR-E i believe.
you can forget about the Q5, which is the older models. all have been updated to the R2. there are also models using cree XP-G R5. the beam characteristics of the R2 and R5 are different. R5 is more efficient, and brighter, but the R2 throws better.
then there is the Q3 version, which I have, and its difference is its neutral tint. less overall output but renders better in my opinion.