To Use REM Refine Mode

Select the region object or primitive object you want to refine.
Right-click and click Refine Mode.
Use the grips to modify the geometry of the object to include or exclude raster data.
Right-click and click Refine Mode to exit from refine mode.
If you select a region object or a primitive, you can switch to Refine Mode to fine tune the geometry used for defining the REM object.

Note: You can only switch to Refine Mode if the REM object is still associated with the image.