The Surface Comparison Utility can be used to add, subtract, or find the mean (average) of the height values of two surfaces.  This utility is commonly used to create a new surface that represents the "deltas" or the differences between two surfaces.  Any two surfaces that are in memory can be selected using the dropdown lists in the top right corner of the dialog.  The location of Surface B relative to Surface A can be adjusted using the sliders.  Click the OK button to create a new surface.