Hello pwilliams18,
The %usagefield% is a variable that is coded to parse the Exchange tracking logs for the date and timestamp of the last time the group received email. After the date and time is harvested from the tracking logs it is then written into the groups extensionAttribute14 field (or the designated field defined in the Group Usage console), so SmartR can generate a report based on the information contained in the Extension Attribute for each group in Active Directory. For more information on using the Groups Usage Service, please review the Imanami Knowledge Base article provided at the link below:
http://community.imanami.com/forums/thread/181.aspx