Global Server Load Balancing (GSLB) regions are used only if the Response Policy is Region Only, to direct traffic to data centers with region-specific content. Add a region to a host on the ADVANCED > GSLB Services page so that traffic that originates in that region is directed to the Site IP address.
A number of predefined regions are listed on the ADVANCED > GSLB Settings page. You can also create a custom region by specifying a region name and then adding one or more regions from a list.