Elctromechanics Mode in v4.3

Hi,

I am trying to understand this example,
http://www.comsol.com/showroom/gallery/444/

The 3.5a version and 4.3 version look quite different.

I have several questions about the 4.3 version that has the Electromechanics mode,

1) Why do we need to separately define block2 and block3? since outside the cantilever its all air

2)The symmetric boundary5, what is that for? Why can't we just assign the other side “air”?

3) About Prescribed Mesh Displacement:
a) when we clear/check the prescribed z displacement, what does that mean?
b) Since “Prescribed Mesh Displacement2 “ and “Prescribed Mesh Displacement3 “ do the same thing, just plying to different boundaries, why can’t we just combine them?

Thanks in advance.

Best,
Yemin