Choose the Mask... menu item to invoke the DTM Screen
  Editor, which enables you to define areas on a
  DECwindows benchmark image that DTM will not compare
  against the test results when the test is run in a
  collection.

  You can automatically read a DECwindows benchmark image
  (if it exists) into the Screen Editor by clicking on a
  DECwindows test in the view window, then pulling down
  the Testing menu and choosing the Mask... menu item.

  The Screen Editor operates on DDIF files only.