In this system, a separate plugin acts as tuning co-ordinator, and all instruments that are compatible connect to it behind the scenes. It's a huge time-saver when working with multiple instruments at the same time, since you don't need to load tunings into each one. Also, tuning can potentially change dynamically on-the-fly, which is very convenient in many cases.

It's an open-source system, so any dev could make one of these master tuning plugs. But the original one from ODDsound (who developed the system) is seemingly still the most capable one: https://oddsound.com/

This one is good too: https://infinitone.com/

The free synth Surge XT can also act as MTS-ESP source. It doesn't do any dynamic scale changing, but at least it can save you the repetitive scale loading: https://surge-synthesizer.github.io/

This one is also free and is promising, though it's still in beta and a bit crash-prone: https://wilsonic.co/

Hope that helps!

I apologize if this has already been answered, but I couldn't find it.

I know that the Stage version can't set a custom scale from the user interface. But I wonder if it can receive scale and tuning information from a host (DAW or notation program).

I know some program, for example Logic or Dorico, can send tuning/scale information to plugin instruments that can understand it. But I don't know if Pianoteq can do it.

Paolo