The notch blend is used to keep a blend on a "shelf" when you move the faces near it. In the image below I have a solid body and do a move face and you can see the blend want to move along with the face to continue with the curvature. In the second image the pink blend is marked as a notch blend and then when I do the move face the blend will run out to the radius and then not continue past the point of tangency to the wall. A notch blend is primarily used in aerospace parts.
Top - Not notch blend Botton - Marked as Notch Blend