I followed discussion here: In a constant speed propeller, why does the RPM drop with a prop pitch increase?, I got confused about the RPM there. As the opening statement said in that discussion, "When a constant speed propeller changes its pitch from low to high, RPM would decrease."

From that discussion said that the RPM there is not the RPM of the engine that used to power the airplane, but the RPM of the propeller itself, how fast it rotating after the pitch was changed. The more pitch of course will produce more drag and as the product, the propeller will rotate slower than previously.

Please you masters of the aviation, please confirm if it is true, the RPM is the rotation of the propeller itself and not the RPM of the engine/turbine used to power of the airplane. That have confused me for quite long time and made me very difficult to understand many explanation.