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

• uses parts of "libnm-core", that are public API of "libnm"
• can be statically linked into users of libnm-core (like 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 can also be used without full libnm.

That means:

• you can use it everywhere where you either statically link with libnm-core, or dynamically link with libnm.
• you cannot use it inside libnm-core itself. This is the difference between nm-libnm-core-intern and nm-libnm-core-aux.

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