Convert a subdivision component selection
You can convert an existing selection of subdivision components to another component type. This saves you additional work in having to drop your existing selection and then selecting the other component type.
For example, if you have already selected some edges on a polygon proxy mesh, you can convert the selection to the faces associated with those edges so that you can then modify the faces with the polygon editing features.
You can convert the selection to faces, edges, vertices, and UVs. In addition you can expand a selection to select additional components that surround the existing selection.
To convert an existing subdivision component selection to another component type
and 
on an
edge of the surface and select the component type you want to convert
to from the marking menu that appears.