I have been wondering what the difference is also.
I had an AR suppressed in 300 Whisper. It is an ok round, I shot it alot but subsonic the ballistics were like lobbing a rock. I got bored with and sold it finally Personally it one round I don't miss.
It was pretty quiet though, comparatively speaking. It probably would make a good subsonic CQB round but I was shooting 100 yards and beyond with a 240 gr bullet at 1050 fps
I really had to know my bullet drop at 150 and 200 yards