Webtrends Analytics 9 Adminstration Guide, November 2012
If you want to create reports that track sub-items within a primary category of data, you can do so by creating a drilldown dimension and including it in a custom report. With custom drilldown, report users can start at a general level of data and drill down to reach more specific layers.
For example, if you create a report that provides information on the electronic equipment purchased from your site, users may want to see information about popular brands of electronics as well as learning which models of computers, televisions, and DVD players customers purchased. By including multiple dimensions within a single drilldown dimension, you can provide this information.
To use drilldown data, create a custom dimension, select Custom Drilldown in the Based On dialog, and create a list of the dimensions you want to make available in the drilldown. You can include up to ten dimensions. Custom drilldowns are limited to hit-based dimensions based on query or cookie parameters.
Drilldown reports have the potential to generate very large amounts of data and use a great deal of memory, so you should carefully consider how many unique combinations you could be generating when you combine dimensions in a drilldown report. Because each dimension has the potential to contain many unique elements, keeping a small number of dimensions ensures your desired results are not trimmed or reach their table limits too soon.