The host controller sets the "serverip" variable in the flash of an AP. On an AP, you can automatically allow it to discover a controller via DNS, DHCP Option or ADP (broadcast/multicast). If you set it statically, you can set the "master" variable, or the "serverip" variable or even both to find the controller. The "host controller" portion of the GUI sets the "serverip" variable. If both are "master" and "serverip" are set:
"On the AP, the other end of the GRE tunnel is specified by the IP address configured variable values (in descending order of priority) <master>, <servername>, and <serverip>. If these variable are left to default values, the AP uses DNS to look up aruba-master to discover the IP address of the controller. "
Why are both of these variables available? Way back in the day, it was required to point an AP at a local switch to terminate his GRE or IPSEC tunnel and upgrade its firmware (serverip variable), but still let the AP know who his master controller is to send statistics to. You no longer have to set both variables manually, so just setting the master variable is all that is needed. The paragraph above describes what is done if both variables are still set from back in the past.