Cambridge dictionary for "id":
abbreviation for identification: any official card or document with your name and photograph or other information on it that you use to prove who you are:
Do you have any ID? A driver's license or check card will do.
I mean that argument is ridiculous, saying that things are "documented" when the thing is literally called tmpfiles.d and the man page starts with the following explanation:
So basically some genius decided that its a good idea to reuse this system for creating non-tmp directories. Overall my opinion of systemd is reluctant acceptance though I always wondered why the old way was a problem. Need a service started on boot? Well, we had crontab and sysvinit with some plain files. Need a service shut down? Well that's the kill command. I guess I don't really know why systemd was made