Commands to modify the polylines that form a cross-section. 

From left to right the buttons are:

Insert vertex: Insert a vertex at the point selected. It must near an arrow. 

Delete vertex: Delete the vertex selected. It is recommended to have the polyline selected to see where the vertices are. 

Delete range of vertices: Two vertices are marked and all vertices between these two are deleted. 

Delete vertices window: A square is selected with the mouse and the vertices within it are deleted.