Subsonic
I think there is uncoupling and true structural aerodynamic independence. At low Mach, the speed at which air becomes compressible, as I recall is ~ 269 knots. Below that value, air is considered inviscid, and is free to interfere with solids located close by (twin tails). I was taught that interference (airframe) is similar to Ground effect, only at altitude. Shock waves are less mysterious (to me) and since they are frequently visible succumb more easily to explanation.