When you view a report, you can filter the report data by zone. In the zone drop-down filter, report services lists each zone by its full zone hierarchy, so that you can choose based on parent or child zones. For example, if you have a child zone named California as part of a parent zone West which is part of the parent zone United States, the zone appears in the list as "United States/West/California.
Zones are listed in the zone drop-down filter in alphabetical order, and the first zone in the list is the default zone. When you first open a report, report services initially generates the report data based on the default zone.