You can select at this step of the Log wizard which variables of the web query and which cookies you want to extract in new custom fields
For example if you have some kind of campaign that makes visitors lands on your web site with an URL like this:
You can extract the CampaignId variable in a new field so you will be able to identify very easily all visitors sent by a specific campaign.
The new field will be extracted from the UrlQuery field. In the mentioned example the value of the UrlQuery field would be "CampaignId=%Number%" and then the value of the extracted CampaignId field would be just %Number%
Some predefined query parameters can be extracted by just selecting an option in the Analysis settings so you don't need to add those here.
If you use a specific cookie on your web site, you can add it in the list of cookies to extract so you will be able to easily filter all web requests according to a specific value of the cookie.
The source field from which the new field is extracted is Cookie.
The ASP.NET and PHP session cookies can be extracted by selecting an option in the Analysis settings so you don't need to add those here.
Result with the two additional columns CampaignId and MyCookie