[dnbgfher]

Nah, mostly it's just (IMO) the result of AdaCore being needlessly confusing. AdaCore also maintains the GNU compiler, which is what their Pro offering is based on. I'm not really sure why the Community edition exists, as it's basically at most a slightly different version than the GNU offering.

[johnisgood]

Someone showed me that GCC Ada supports older versions of Ada, while AdaCore's Community edition does not. There are other differences of which I am unaware, but I found this difference to be a particularly major and surprising one.

[dnbgfher]

I'm not sure what you mean. Ada hasn't ever undergone any breaking changes I'm aware of, so even Ada 83 would be valid Ada 2012. I'm not sure how a compiler could lose support of older versions.

The only difference I've ever run into was an AdaCore-defined (as in not part of the standard) aspect that was known but not fully implemented by GCC Ada, while the Community edition did have it. That's since been added to GCC as well.