Search notes:

/run/user/$uid

/run/user/$uid is created by pam_systemd and used to store non-essential runtime files or objects, such as sockets, named pipes, keyring daemon files, pulseaudio files etc.
This directory is owned by the user whose uid is $uid.
The permission bits for this directory need to be 0700.
The directory is created when the user (with $uid) logs in and is destroyed when he logs out again.
The environment variable $XDG_RUNTIME_DIR might(?) point to this directory (compare XDG)

See also

Prior to systemd, the files that are now stored in /run/user/$uid were typically stored in /tmp.
Using /home/_username_ was not practical because this directory was often mounted over network filesystems.
/run

Index