I don't think I like this as a replacement for simply defining terms in the text. You could do both but something like "S a subset of Range(X)" when the equatio

I don't think I like this as a replacement for simply defining terms in the text. You could do both but something like "S a subset of Range(X)" when the equation is about the probability that X is in S doesn't seem important enough to take that prominent a position in the equation. I can see this style of annotation it being helpful when you need to add some context or want to explain a pre-existing formula. And I can definitely see this being used to highlight key ideas during a tricky derivation, stuff like partial integration could be a lot neater if you can point out the part of the equation that you're partially integrating.