BTs - scripted action nodes

Hi all!
(BT = Behavior tree)

If you work with some game engine (not UE4) which have stock BTs, could you ansswer me the following:

  1. Does engine supports sripting BT actions? I.e. not requiring changing some program language code,
    but actuall scripting intended for designers/animators?

  2. If does, what it looks like? Is it just sequence of operations with parameters or something else?

  3. Is there some design decision thesis you remember about the implementation?

In Unity BTs are implemented on C# level, so they are scripts itself.