NetworkManager provides a client library libnm.
NetworkManager core does not (dynamically) link against all of libnm.
Instead, it statically links against a part of it.
That part is the static helper library libnm-core-impl.
libnm-core-impl implements (and provides) the API from
libnm-core-public, which is part of the public
API of libnm. In this form, libnm-core-impl is part
of the implementation of libnm. It also implements (and
provides) an internal API libnm-core-intern which
can only be used by those who link statically against libnm-core-impl.
Only NetworkManager core and libnm are allowed to statically
link with libnm-core-impl. Consequently, only those are allowed to include
libnm-core-intern.
This directory should not be added to the include search path of other
components as they are only allowed to include libnm-core-public
and libnm-core-intern.