This page has info removed from UniFi Cloud Controller to make that page more concise.
Q: I keep reading the same thing in the product description for Unifi: “Multi-site management capabilities, enabling a single controller to manage distributed deployments and multi-tenancy for managed service providers” but I can't find any documentation on how to actually implement that last part.
A: It is different sites within the controller.
Is this what you are looking for?
Also somewhat related:
In many deployments where it's not possible/desired to have controller running at the premise, you can run the controller in the cloud or your NOC.
UniFi AP has a default inform URL http://unifi:8080/inform. Thus, the purpose of using DHCP option 43 or DNS is to allow the AP to know the IP of the controller.
You can either configure DHCP server, DNS server, or simply use the UniFi Discovery Utility to make all local APs inform back to the controller. You can even just set the inform address by SSH'ing to the access point and running: set-inform http://ip-of-controller:8080/inform