Changes the existing polygon component selection to the vertices that are associated with the components originally selected.
For example, if you have a polygon mesh comprised of four-sided polygons (quads) with several faces selected and then choose Select > Convert Selection > To Vertices, the selection is converted to the vertices associated with the previously selected faces.
Many of the convert selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.