You wish to learn more about sub-hostnames. You see the field in Events -> Settings and do not understand how to use it.
When managing multiple subscriptions, a sub-hostname is used so that each subscription resides on a unique URL. It becomes part of your URL like so: http://hostname.subhostname.dojiggy.com. When more than one active event is present, all events require a sub-hostname, but you can still promote the default event as http://hostname.dojiggy.com - with no sub-hostname. The Default Event field is set from Events -> Settings -> Advanced Settings.