The 'Group Domains' parameter allows you to group individual hostnames into their respective domains. The value specifies the level of grouping to perform, and can be thought of as 'the number of dots' that will be displayed. For example, if a visiting host is named cust1.tnt.mia.uu.net, a domain grouping of 1 will result in just uu.net being displayed, while a 2 will result in mia.uu.net.
The default value of zero disables this feature. Domains will only be grouped if they do not match any existing 'Group Site' entries, which allows overriding this feature with your own if desired.