Reason: The current supported/partial/hardcoded split is not detailed enough and can be misleading.

Should default to /usr/local/share:/usr/share.List of directories separated by : (analogous to PATH).pam_systemd sets this to /run/user/$UID.Should not store large files as it may be mounted as a tmpfs.Can only exist for the duration of the user's login.Modified every 6 hours or set sticky bit if persistence is desired.Filesystem fully featured by standards of OS.Must be owned by the user with an access mode of 0700.Not required to have a default value warnings should be issued if not set or equivalents provided.Used for non-essential, user-specific data files such as sockets, named pipes, etc.Where user-specific state files should be written (analogous to /var/lib).Where user-specific data files should be written (analogous to /usr/share).Where user-specific non-essential (cached) data should be written (analogous to /var/cache).Where user-specific configurations should be written (analogous to /etc).See Environment variables#Globally for information on defining variables.

It is up to the user to explicitly define the other variables according to the specification.

Only XDG_RUNTIME_DIR is set by default through pam_systemd. This section will attempt to break down the essence of what it tries to achieve.

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