What diffferent LED's versions/generations are there out there?

I have not got the hang of this really.
I see Q5, T6 and U2 but do not know the difference.
I assume there are more than these three but as I said I do not know……
Is there a FAQ or a good thread explaining this……

It might help if you look on some of the seller sites like Mouser, Cutter etc. there’s a lot to grasp and I don’t know it all. The Q is xre or XPE, the R is for xpg, T and U are XML
Number is Second /

Number is first /
Now whe you see 1a 1c that’s color tint and a whole different ball game

(Going on the Cree site may help too)

This page gives data and pictures of most LED’s used in flashlights. There is additional information for each manufacturer, for instance Cree: