Many thanks Spremkumar, but I already have this document & it still does not Explain what YT or YG mean?

SURELY THERE IS A DOCUMENT SOMEWHERE THAT DEMYSTIFIES YOU'RE NAMING CONVENTIONS FOR ALL NAMES? IS THIS TOO MUCH TO ASK?

I believe that a 'Y' (or X or Z for that matter) designation on an IOS release indicates that it was purposefully branched off the main release train in order to support a specific feature and/or bugfix. That branch can go through several iterations before the test code is integrated back into the next release of the main train. It basically gives the IOS developers a way quickly release something w/o impacting the entire train.

For example, I know there's a 12.4.11T-XJ release that has a wireless feature I've been waiting ages for. That feature is ONLY in that XJ release, but should be officially released in 12.4.12T (or whatever the next major release is.) I'll put that release on non-critical and/or test-lab routers, and mess around with it until it gets 'officially' released.

If you don't have a TAC engineer telling you to use a such a release (e.g. because it solves whatever problem you're having) then stick with a either the ED (Early Deployment) or GD (General Deployment) release.