From W3C Wiki
The "As'Used'In" approach says: make a web page which uses a term for the Resource in a highly-constraining way (one might even say "defining it"). Then construct a URI which comprises the URI of the page with the in-page name of the Resource.
One syntax for this is HashURIs, but there are others.
We can imagine a URI scheme like
In either case, the semantics are something like: "the thing call FOO in document BAR".
There's a huge open issue about whether using an AsUsedIn identifier constitutes endorsement of the original document. It's annoying if it does, but what is being identified if it doesn't?