I think the answer should be "it depends".
With a mechanical tacho, the thing normally just counts revolutions. With an electric one, it normally counts engine running time, but it could be running time above a certain RPM.
You can also bring in airspeed (to measure just airborne time), retractable gear status (again to measure just airborne time), oil pressure (to measure engine time but only above e.g. idle) etc etc. All these and probably others have been done.