If you imagine that you (Bill) have organized a football league that has three teams.
So if someone wants to find the phone number of Steve on team A they contact Bill who contacts the manager of Team A (John) using the phone number returned by Bill and John tells them.
In the Diagram below I have modified Bills list to include Barry.
We also need to add a note in Johns list to include Barry as he needs to send him the list and list updates. Note: Primary zones are now called master zones and secondary zones are now called slave zones.
So now you have three lists but who manages the lists?
Well each team has a manager so you let the manager handle the list for the team.