What is the meaning of "-x-icu" in PostgreSQL's "collates"?

I had this query: SELECT * FROM table ORDER BY label ASC; Since the labels are not in English, they didn't get sorted in the right order (ones beginning with "ö" were not in the bottom/end). I therefore tried: SELECT * FROM table ORDER BY label COLLATE "sv-SE" ASC; SELECT * FROM table ORDER BY label COLLATE "sv_SE" ASC; These gave errors about those collations not existing, which baffled me. After a bunch of searching, I figured out to do: SELECT * FROM pg_collation; Which revealed that it's supposed to be: sv-SE-x-icu It worked when I used that identifier, but what's with the "-x-icu" stuff? What's that all about? I hate that they always have to mess with the standard locale identifiers so you can never rely on just the standard "language_location" format.