When you clip the mesh warp group to the curve, the visible extent of the group is determined by the containing curve, but the invisible parts are merely hidden rather than being removed. If you use the curve to mask (or crop) the mesh warp group, you are cutting off the parts of the group that lie outside the containing curve, albeit non-destructively. The difference is particularly noticeable with the 3D Effect: a masked/cropped object will have the effect applied after masking/cropping, makin