Select a Limit to of Boundary on the Toolpath Limit dialog to resitrict the active toolpath to a specified boundary. Select the Boundary from the drop-down list, select a Projection Plane of Workplane, View Plane or Best Fit Plane, and then click Apply.
Original toolpath:
Limited toolpath:
Note: This result is the same as when you use Limit Toolpath to Polygon. It is up to you which option you use. Sometimes Limit Toolpath to Polygon is easier/faster to use than Limit Toolpath to Boundary. Sometimes Limit Toolpath to Boundary is easier/faster to use than Limit Toolpath to Polygon. Sometimes there is really no difference between the two.