It can be quite confusing to determine the proper IOS to download. Often, customers call TAC just for that reason.
Different Tracks are created depending on customer needs. The mainline track offers the most stable IOS of all tracks but it does not provide the latest and greatest technologies that Cisco has to offer. When you introduce latest and greatest, you often find that things don't work as expected. Usually that's the 'T' track.
I'm not saying the T track shouldn't be used but if you have all the needed services out of the mainline track, you should stick with mainline for better uptime and router reliability.
Those other tracks listed in your initial posts are the odd tracks. There are often created when a group within Cisco is coming up with a technology that isn't part of the T track nor the mainline track. I would only use those odd tracks if you need the services they are offering.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...