What ACARS reports depends on the 'contract' set up with the recipients signed onto the ACARS reports. The pilot may have no idea what is contracted by the ground systems. If they ask for every waypoint, every vertical speed higher than n fpm and every drift off track more than x.y nautical miles and every change in the active route - they will get that and the pilot will not know that is what has been contracted.
Once again, ACARS DOES NOT DO POSITION REPORTS WITHOUT BEING LOGGED ONTO ADS!!!! The ground systems cannot setup a contract without the aircraft logging on.
If the pilots haven't logged the ADS onto the FIR ADS address the ACARS will not send position reports. ATC cannot log on to the aircraft without the pilots requesting it first as far as I know.
There seems to be a common misconception throughout this thread that ACARS is sending back position reports constantly. It doesn't.