The metrics included with Understand have evolved over many years to accommodate common customer requests. However, we recognize that you may need something different. To help you develop custom metrics there is a full API interface to the Understand database as well as a fully developed plugin mechanism.
For a full description and definition for these metrics, Open Understand and go to Metrics->Browse Metrics and then click, “Metrics Definitions,” in the top right corner of the metrics browser.
Here is a pdf containing a list of all metrics and a description of how they are calculated.