Ok my bad. My sentence "3 types of RA advisory" should read "3 types of TCAS advisory". Thanks FlightDetent.
I have always thought that it is closure rate because I've once or twice receive a "Traffic Traffic" TA advisory with ND showing TA intruder in an amber circle. Traffic was right below me 1000ft in RVSM airspace, but since both aircraft were maintaining level flight TA didn't resolve into RA (no closure rate). Furthermore from Airbus' FCOM 1.34.80;
-...the TCAS determines for each intruder its relative bearing, its range and closure rate and its relative altitude if available (ATC mode C or S).
So it's probably both the distance and rate of closure.
Does anyone know if TCAS2 and T2CAS is the same?