This could be either a trivial question, or an arcane mistery. For me it was the second one, until I did some research and I found out that the answer was quite easy 🙂
So, how does cfengine know what's your policy hub, once you have bootstrapped a client?
The surprisingly simple answer is: by reading the contents of $(sys.workdir)/policy_server.dat, usually /var/cfengine/policy_server.dat
Good to know, when you are planning to point a client to a different hub for any reason 🙂