I'm trying to understand how PCI segment(domain) is related to multiple Host Bridges? 

Some people say multiple PCI domains corresponds to multiple Host Bridges, but some say it means multiple Root Bridges under a single Host Bridge. I'm confused and I don't find much useful information in PCI SIG base spec.

I wonder

(1) Suppose I setup 3 PCI domains in MCFG, do I have 3 Host Bridges that connects 3 CPUs and buses, or do I have 3 Root Bridges that support 3x times buses but all share a common Host Bridge in one CPU?

(2) If I have multiple Host Bridges(or Root Bridges), do these bridges share a common South Bridge(e.g., ICH9), or they have separate ones?

I'm a beginner and google did not solve my problems much. I would appreciate it if someone could give my some clues.