+ unit in systemd.
+ See <filename>/proc/self/mountinfo</filename> description
+ in <citerefentry><refentrytitle>proc</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+ </para>
+
+ <para>Some file systems have special semantics as API
+ file systems for kernel-to-userspace and
+ userspace-to-userpace interfaces. Some of them may not
+ be changed via mount units, and cannot be disabled.
+ For a longer discussion see <ulink
+ url="http://www.freedesktop.org/wiki/Software/systemd/APIFileSystems">API
+ File Systems</ulink>.</para>