Hi and welcome to the community
If you upgrade from PT11 to PT12 you will have the PT12 version with updates and if there is a major upgrade a.k.a PT13 you would get that free in the support plan. The support plan you pay for will last you 12 months regradless when you upgrade during the year.
So if you upgrade say 14/July/2015 it will be valid until 14/July/2016
After you do this upgrade you will have a "upgrade option" button in your account that you can activate (disabled as default) and that will upgrade the support plan for you when it is time to do so.
Regarding the info on this you could google it or look at the Avid website (if you have a whole week free)
That is a really hard place to find stuff hehe
I can post a few links for you if you want.