Snapping is often used while interacting with the Structure view to make it easier to align the cursor with either existing geometry or at convenient locations in space. By default, the cursor snaps to the intersections of the working plane grid lines as shown in the following figure.
You can snap to vertices, midpoints, centers of faces and projection lines. Right-click an object in the Structure window and choosefrom the context menu to alter which snap points show. You can toggle snap points for edge vertices, edge midpoints, and face centers.