Webtrends Analytics 9 Adminstration Guide, November 2012
Webtrends Analytics 9 campaign reports rely on Visitor History to associate a visitor's behavior with the visitor's access to a campaign. For example, if a visitor requests the landing page for an advertising campaign, you know the visitor has seen the advertisement and clicked through to your website. But when the same visitor returns to the site the next day and makes a purchase, how does Analytics 9 know to attribute the purchase to the campaign?
Visitor History makes this information available by connecting information about visitor activity over time. When you identify your campaigns with the WT.mc_id parameter, Webtrends Analytics 9 automatically generates the WT.vr.rac parameter for subsequent hits within the visit and converts WT.vr.rac into a series of campaign-specific parameters that in turn provide the data for campaign drilldown reports.
Visitor History also allows you to make campaigns expire after a set number of days. By default, reports credit site activity to the most recent campaign identified by the WT.mc_id parameter. In other words, a campaign does not expire until a new value for is WT.mc_id passed. However, you can choose to attribute activity to each campaign for a limited time. For example, if you use short-term, targeted campaigns, you may only want your reports to show hits and visits attributed to each campaign for 7 days after it launches.