From what I've read and had some classes about, ADS-C is like a separated dedicated link.
It will generally be used, for example, for airline operations communications. It will have a fee and will be charged.
It's a system independent from ADS-B in and out, whereas as far as I know, ADS-C is not intended for air traffic and air space management.
Now, that's what I hear, which could be proven wrong.