I’ve used both game maker and briefly game maker: studio. Studio was a complete rewrite of the engine. It has different features, different language standards and syntax, and is considered a separate product. As a result many people still use game maker version 8.1 or older which predate studio. Studio is also a compiled language whereas pre-studio was interpreted. As a result considerations in performance matter a great deal more in one versus the other. Similarly unlike most engines where functions might be removed or added this rewrite changed a large amount of behavior.
I imagine this discussion was had in the past but should we separate the tags and have two?
Since an answer has already stated that this is a thing to be done, I am making a list of questions here that should have the studio tag. Since some features do transfer over it might not be necessary to squabble over questions that just happen to be involving Studio; however, as I am primarily a veteran user of Game Maker 8.0 these are questions that are involving features that were either heavily modified and/or added in the Studio rewrite.
I have only been searching for about 5 minutes so don't expect this to be exhaustive as a list.
I do not have enough reputation to create the tag itself (95 rep short) so if someone would please create the new tag by adding it to these questions I will gladly edit the wiki tag accordingly and begin using it elsewhere.