Multisite in MM is giving you the ability to point to multiple PBX. You can look in the Concepts and Planning guide on the Avaya website for more information. Avaya ATAC also can provide you information on this.
Important note is that you can go forward to activate Multisite but you can't go back if you decide against it. If you decide against it, then you have to rebuild your system.
With Multisite, you have to create a rule list to point to the multiple PBXs and numbering ranges.