Partner Hosted is the image used for systems installed and managed under Avaya's OSS/Powered by Avaya product range. I think its install process is different as Avaya have a method of injected the preconfigured settings for the customers for who the virtual server is being launched.
The CPE image is the one for anyone who wants to install and manage their own virtual server (it means 'Customer Premises Equipment' despite being in the cloud).
I think, but can't swear to it, that the machine you end up with is the same, just the install process differs.
From a quick search it looks like even though the AMI is only listed under N.Virginia, it possible to move the instance to another region after launch.
Stuck in a never ending cycle of file copying.