The nebulousness is one of the reasons as to why we're not stabilizing it!
Sorry what I mean in those two phrases is the same thing, and it's the sae as macro_rules sort of except that there basically isn't any hygiene. It's as if the exact tokens produced are simply copy/pasted into the module, name conflicts and all.
I think @nrc or @nikomatsakis may be able to help out with this. Remember though that this is a slice of stabilizations. We will continue to stabilize more features over time, such as macros in expression positions. Just because they may not be stable in this first pass doesn't mean we're removing support for them.