Some time ago, tags were created for all three of these frameworks, but with no excerpt or other guidance. There are currently 86 questions tagged notifications, 51 questions tagged subscriptions, and 10 questions tagged messaging.
Currently, many of the questions that use these tags are clearly about the relevant messaging framework.
However, not all questions tagged with these are. These three tags are also used to tag questions with requests for help with custom code that sends the user mail or shows the user a pop-up or alert box. In these "other" uses, they are for most of the time used interchangeably for "How can I send my users such-an-such message?", sometimes also for "How do I notify my users about such-and-such event?", and sometimes as a synonym for the totally unrelated (and now also deleted) notice tag.
I believe these tags currently are a mess, and that they need to be given meaningful tag excerpts to stop people using them for things totally unrelated to each other.
After agreeing on their uses, they could also need some clean-up (by retagging the mistagged material - the numbers are low enough to make that a realistic proposition).
I started out by trying to giving the most widely used one notifications a tag excerpt tying it to the Notification framework. That exerpt was promptly rejected, with the explanation:
That's not exactly how this tag is used now. Discuss it on Meta first, if you want to change it's meaning.
A week ago, I made a specific proposal here on meta about I wanted to fix these tags.
This proposal had just two upvotes and one downvote after a week. This is not much of a mandate for going forward with such a project, so I tempted do just let this one slide (i.e. do nothing).
However, before I give up, I'll try to get a bit more feedback.
I've reposted my original proposal as proposal one below this question, and in addition posted the two possible alternative outcomes I can imagine as alternative proposals. I.e. there are three proposals, each in a separate question so that each can be voted upon seprately.
If you have another proposal about what to do about these three tags, please join in the fun.
If you want to preserve the status quo completely unchanged, the proposal to vote for is proposal two.
If you like the status quo, but like to see it documented in some way, as well as redundant tags removed, the proposal to vote for his proposal three.
Edit: Can whoever downvoted all three alternative proposals (including #2, "do nothing") please explain the vote?