/run/user/$uid

/run/user/$uid is used to store non-essential runtime files or objects, such as sockets, named pipes 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)

Apparently, this directory is created by pam_systemd.