In particular, in our own version of the system, there is a directory "/usr" which contains all user's directories, and which is stored on a relatively large, but slow moving head disk, while the othe files are on the fast but small fixed-head disk. — https://www.bell-labs.com/usr/dmr/www/notes.html
Isn’t that just a backronym? I read recently (here, I think) that on some original specific UNIX system, the user directories were there originally, and then other stuff that belonged elsewhere went in there because the other physical volume (where things like /bin were) ran out of space. Later another dish was added and profiles were moved there, to make /home.
