The Left Boundary, Right Boundary, Max Radius Boundary and Min Radius Boundary parameters limit the portion of the feature that will be roughed. These boundaries are displayed in blue whenever you select the rough operation in the tree view. When the material boundaries are defined to machine a part, the boundary must be specified so that it completely encloses the path (e.g., the path cannot start or end in the middle of the rectangular box; it must start on, or outside of the boundary).
When the path is defined, it may extend beyond the material boundary. This is a powerful technique in roughing, since a long path can be defined, then an area in which a specific portion is roughed only between material boundaries can be established. In a second segment, the path could be copied, then the boundaries to rough another section of the path can be redefined, and so on. Any change that is made to a boundary parameter is automatically displayed on the screen.
Click here for graphic explanation with toolpaths