Each compute server is split into two PDomains, where the bottom four CMIOU slots are part of PDomain 0, and the top four CMIOU slots are part of the PDomain 1.
A PDomain operates like an independent server that has full hardware isolation from the other PDomain in the server. For example, you can reboot one PDomain while the other PDomain on a server continues to operate.
You can have from one to four CMIOUs in each PDomain, or you can have an empty PDomain that you can populate later.