If an application is intended to be installed by root on a system wide basis then /usr/share is recommended to be used as value for datadir and /etc/xdg is recommended to be used as value for sysconfdir. In case the /usr/share hierarchy is not writable it is recommended to use /usr/local/share as value for datadir instead.

If an application is intended to be installed by an unprivileged user for exclusive use by that user only then $XDG_DATA_HOME should be used as value for datadir and $XDG_CONFIG_HOME should be used as value for sysconfdir. If $XDG_DATA_HOME is not set, the default value of $HOME/.local/share should be used for it. If $XDG_CONFIG_HOME is not set, the default value of $HOME/.config should be used for it.