The taginfo for api describes what an Application Programming Interface is, but doesn't give any guidance as to what kind of question it applies to.
Given that the majority of code here is at application level, almost all of it uses some API or another (for example, the standard C API, or the Java Beans API), and it would be unhelpful to tag all these with api.
I can see that the tag might be useful for code which exposes functionality by providing an API for programs to use, and where that API itself is a large part of the review, but the existing questions don't seem to match my expectation. A lot of them are about interacting with network services of one sort or another.
Could someone who knows what questions api is actually for please edit it (or explain here) to give some guidance for which questions it should be applied to?