There is just one version. The commercial version just comes with support.
I suppose you can get the support anytime you want. If you didn’t configure it properly, or get something other wrong, it might be more complicated to fix it than taking their support in the first place.
Yes, there are just some legal limits, so you can’t say I’m Nextcloud or the official Nextcloud. There are plenty of Nextcloud hoster, I don’t know how many have bought in official support as well. Would be great if such hosters could at least help with maintenance of some apps or documentation or anything to help out the community.