Webtrends Analytics 9 Adminstration Guide, November 2012
The profile's Visitor History database maintains a history of unique visitor activity over time. You can optimize the performance of profiles that use Visitor History by enabling visitor history trimming and configuring trimming settings to retain the visitor data most valuable to your business.
You can qualify the visitors to trim based on their number of visits, lifetime value, visit recency, and purchase recency. Keep in mind that if trimming is not enabled, the memory required to process a Visitor History-enabled profile may not be sufficient, which prevents Webtrendsfrom analyzing the profile.