nm-libnm-core-intern is a static library that:

• uses parts of "libnm-core", that are public API of "libnm"
• that is statically linked into libnm-core (and thus libnm and NetworkManager).
• that can also be statically linked into other users of libnm.

Basically, it is a static library with utility functions that extends libnm-core (the part that is public API of libnm), but it is used by libnm-core.

That means:

• you can use it everywhere where you either statically link with libnm-core, or dynamically link with libnm.
• you can even use it inside of libnm-core itself. This is the difference between nm-libnm-core-intern and nm-libnm-core-aux.

Also, since nm-libnm-core-intern itself only uses public (stable) API of libnm, you theoretically can copy the sources into your own source tree.