The edge guideline aids construction by automatically determining the edges present in a 3D view, and shows a preview of the guideline as soon as the 3D cursor comes within snap radius of one of the edges. A left mouse click then creates the guideline. Edges are not only the ‘proper’ edges of a 3D solid, but also the edges of the triangles, which define a 3D solid in a visualization. An exact rectangular surface is made up of two triangles, which means that with the edge guideline you can also automatically enter a diagonal guideline.
You can work with the edge guideline much more accurately, especially with complex bodies, if you switch the representation mode of the 3D view to wire frame.