Start Point is the point where the tool can position itself before moving into the part. Beginning point is the point closest to the endpoints in the geometry that you want to cut.
We do this because sometimes you may want to get into position( start point ) but the closest endpoint on the geometry is not where you want to begin cutting and so you need a 2nd point (begin point) to specify where to move to from the start point to begin cutting.
The start pt is always outside the part...the begin pt is always on the part.
Hope this helps!