The Crop Mesh tool 
on the Cloud/Mesh Tools Tab is used to trim or crop a
Mesh to a drawn rectangular region. Click to start a region in the 3D Viewer
then click again to close the region and crop. The crop will retain the
portion of the Mesh within the rectangular region and discard the rest. This
action is undoable.
If multiple Meshes are visible, the crop will act on only the visible Mesh(es) that are intersected by the region.