Geo Fence is basically a rectangular area that can be seen around a route, in the Service Units tab. There is no way it can be configured. It is a logic built in to the DispatchTrack program.

However, when you click on Geo Fence in the Admin tab, what you get to see is settings for being alerted about a Geo Fence violation, if the Geo Fence Violation Alert has been enabled in the Alerts link.