Hi CHI-WEI C,
1) In Lesson 5 - Setting a Switch using a Wwise-Type, you learn that Wwise-Type properties do not have to be in the same script as where you set it from.
The purpose of this exercise is to not let yourself be limited by handling everything in one script, but rather take advantage of making a system of scripts that interact with each other.
For instance, you could make one script for all switches, that looks something like this...
... but this example requires a reference to the surface, rather than just having a script on the surface object, that acts as a "label" of what Switch to set. Like this.
This may not be the best solution for every game, but it's good to know that this is possible, especially as referencing objects across unity scenes are not supported.
2) Yes, all material shows best practice, which is to use Switches on a per-game object basis, but it is still "possible" to use them for global purposes.
That said, I see how the question can seem confusing, so we'll see what we can do about it for the next update. Thanks!
I hope it clears it up.