Have I missed this somewhere?

You are allowed one region per server. Does this mean:

1. it is illegal to install another there

2. you must overwrite any region already installed when you install a region

3. you cannot rez a region where a region is already rezzed

4. rezzing a region trashes any extant region in the server

Comparison with Unique cards suggests option 4.

V.

Page 13 of the rulebook:

Note: The Corporation can only have one upgrade with the

region subtype installed per server or server root, as listed in

the text box of these cards.

Only one region installed per server, so 2.

Unique cards are different in that there can only be one active at the same time.