At this step you can specify the field you want to use to track visitors of the web site. By default it will be the client IP address but if the web site uses a specific tracking cookie and you configured the extraction of it in the previews step, you can specify it here.
Then you can specify which fields you want to copy the value from the landing request to all successive requests and you can specify a prefix for these new fields.
The result will then look like this
The fields %Prefix%_UrlPath and %Prefix%_RefererSite are copied from the first web request done by the visitor.
The field %Prefix%_EventTime will tell you when the visitor landed, the field %Prefix%_SinceDays will tell you the number of days since he landed and the field %Prefix%_RequestNumber will tell you how many requests he did since then.
So this feature will allow you to easily know from where a visitor accessing a specific web page came from and on which page he landed, when he landed and how much activity he did since then.