Not being a candlepower forums guy, I'm curious if anybody can point to a "laymen's" or "KISS principle" site for Cree tech timeline or tech-hierarchy???

I notice the following:

XM-L T6
XM-L
XM-L2 (I understand this to be the latest tech?)

And the $20 amazon dual-lamp versions to be XM-L U2. (U2?)

It's enough to drive a person bat ***** crazy if you read too long.