In very basic terms ,it can be part of the CPDLC or Controller Pilot Data Link Communication. Or it can work on it's own.
Now this is a very basic description but it can download a full position report automatically on crossing a way point via ACARS +VHF/SATCOM without any pilot interaction after loggin on.
Used in many non radar environments, say North Atlantic Tracks, North Canadian Flexi Tracks, Mumbai FIR etc etc so we don't need to call on HF every 10 degrees. ATC get out report automatically.
In an emergency it can continously send the aircrafts postion if it's in distress.
So an automated datalink position reposting system...maybe.
ADS-B broadcasts it's info like Mode-S via the transponder, so it's range is limted and it's a one way broadcast. C or Contract, 'logs-on'.