3dvolreg's help has some information.
-maxdisp = Print the maximum displacement (in mm) for brain voxels.
('Brain' here is defined by the same algorithm as used
in the command '3dAutomask -clfrac 0.33'; the displacement
for each non-interior point in this mask is calculated.)
If '-verbose' is given, the max displacement will be
printed to the screen for each sub-brick; otherwise,
just the overall maximum displacement will get output.
** This displacement is relative to the base volume.