VDI desktops in a XenDesktop environment need to be able to register with a controller or multiple controllers so they can be managed by the broker and allow connections by clients.
In XenDesktop 4 the default was having the controller information held in an Active Directory OU. During installation you specified an AD OU and the controller installation added the AD objects to the OU so the Citrix Virtual Desktop Agent (VDA) that is installed within the guest OS could find the correct delivery controllers.
This has changed in XenDesktop 5 where the controller information isn’t by default added to AD and the client VDA is configured with the DNS names of the controllers (best to have at least two for redundancy) in the following registry key:
In this example my controllers are lonxd01.lab.int and nycxd01.lab.int
I think storing the controller information in AD which was the default in XenDesktop 4 was a great way of doing things as it gave you one less client VDA configuration setting to manage. If the controller information is stored in the registry and you need to add or remove a controller you have to reconfigure every client which can be a lot of work. If the controller information is stored in AD you can amend the setting in AD and all clients will be able to find the new controller without any VDA configuration change.