storage spaces enclosure awarness

Hi,
As It can be found to have enclosure awarness working it is necessery to have minimum 3 enclosures.
I cannot found requirements for 3rd enclosure, except this fundamental.
So lets assume we have two identicall enclosures full of disks (A and B).
We want to have storage two way mirrored, having one copy on A enclosure and second on B.
And we want to make our configuration be ensclosure aware.
So We need to add 3rd enclosure.
And at this point I have some questions:

• does it need to be identical enclosure or any (passing minimum requirements of course)?
• should it contain identical set of disks or no?
• if not what are requirements for disks (number, sizes etc), what inimum number, size?
• if identical, how those disks will be used? will expand storage size for users or be 3rd copy of data

As I understand 3rd is for quorum or something like that so should be not disk consuming.

Bests,

Artur