Search notes:

/run

/run contains run-time variable data that describes the system since it was booted. Apparently, the directory and its subdirectories also expose userspace sockets and files.

/run/utmp records users that are currently logged in.

See also

/var/log/wtmp, /var/log/lastlog

/run/initramfs, /run/shm, /run/systemd, /run/user/$uid

/run/WSL

The Debian Policy Manual requires /var/run to be a symbolic link to /run (section 9.1.1).

This manual also specifies that /run is cleared at boot time (section 9.1.14).