Take a look at the help documentation for Diffraction Encircled Energy, in particular, the section on evaluation at surfaces other than the image surface:

The diffraction encircled energy is not computed directly on the user-selected surface, but rather on the paraxial focal plane for this surface.  In other words, the ray bundle passing through the user-selected surface is allowed to come to its best paraxial focus (which will be a virtual focus if the ray bundle is diverging).  Then the diffraction encircled energy computation is carried out in that focal plane.  However, for the geometric encircled energy, the analysis is performed directly on the user-selected surface.

Note that you can always just defocus your image plane and see the impact using Diffraction Encircled Energy.  However, once you change the surface setting to anything other than the image plane, OpticStudio will automatically apply the paraxial refocus (for PSF, MTF and Diffraction Encircled Energy analyses).