The Last Offset is the distance from the finish pass to the edge of the shape being cut. This can be any numeric value, but if it exceeds the XY Step value, the cutter will not contact the part on the finish pass. Typically this is set to the radius of the tool in a rough-cut, finish-cut scenario. You can leave some material on the part if you want to clean up the part with a finish cutter.
Last Offset distance on Multi-Pass Outside tool path.