https://en.wikipedia.org/wiki/Ontology_(information_science)
enjoy the rabbit hole!
If I understand correctly, ontology is about categorizing / defining / mapping relations between concepts. No surprise projects working on this exist.
My question was about the 'indexing' step from there: concept -> number to identify it.
So... it seems no universal standard there?
https://en.wikipedia.org/wiki/Ontology_(information_science)
enjoy the rabbit hole!