This function is useful for imported CAD files that contain entities with unrealistic large Z coordinates. The drawing is essentially a 2-dimensional drawing, but seen in elevation view, one or more lines may extend beyond the main plane of the drawing. If the entity is far from the origin, the drawing may seem to "disappear" when seen in a view other than Plan View.
Implementation: Select Tools - Flatten Drawing Entities. You will be asked whether you are sure you want to do this. Click Yes to continue, and all drawing entities are "flattened" to the Z = 0 plane. Click No to cancel the command.
Note: This command cannot be reversed with the Undo (or Ctrl-Z) command.