The VAST-xt is an active scanning sensor while the VAST-xxt is considered passive. The active sensors have force generators inside which act like drives.  The passive sensors rely on springs inside to create the measuring force.  Active scanning is famous and exclusive to ZEISS.  It allows for faster scanning speeds while maintaining accuracy and also long stylus lengths up to 1000mm!  The VAST-xxt passive sensor is standard on the Duramax and O-Inspect in a straight down direct connection.  It can also be attached to the RDS rotating head on machines like Contura or Spectrum.