When a remote desktop deployment is launch through CAS, the itopia automation software creates a default remote session collection which allows users to log in to the session servers. Part of that process is also creating an OU structure that will place servers in specific OU's so policies are applied to the session hosts, a share is created to house the user profile disk, and a security group is created per collection that allows access to the collection.
When you enable multiple collections in an existing deployment, the OU structure will be modified to include a new OU for each collection and new folders are added to the user profile share in order to configure separate shares for each collection. This last part is done because as long as a broker is hosting multiple collections, it will not allow you to use the same share for multiple collections. The additional OU's are created for both organizational purposes as well as for management of the new session hosts through policy. Access to the collections is managed through a security group.
It is important to know that when you switch a user from a collection to another, the user profile disk is created new on for the new collection the user is logging into. The experience for the user would be as if they were logging into a brand new computer without any application configurations or any data they might have previously had in their personal folders (desktop, downloads, documents, etc). When moving a user from one collection to another please plan accordingly and keep the user profile disk in mind.