UmljaGFyZCB3cm90ZSB1cCB0aGUgaHR0cHM6Ly93d3cudzMub3JnL2NvbW11bml0eS9zY2hlbWFi
aWJleC93aWtpL0lkZW50aWZpZXIsIGJ1dCBJIGRvbid0IHJlbWVtYmVyIG11Y2ggZGlzY3Vzc2lv...
Richard wrote up the https://www.w3.org/community/schemabibex/wiki/Identifier, but I don't remember much discussion of it.
The abandoned "info" URI effort leaves me skeptical that non-HTTP URIs can be systematically described in general. I'm also skeptical that individual identifiers of any kind need to be described inline with instance data.
http://info-uri.info/
Jeff