Changes the current selection to any polygon edges that can be selected depending on what components were originally selected.
For example, if you have a polygon mesh comprised of four-sided polygons (quads) with several vertices selected and then choose Select > Convert Selection > To Contained Edges, only the edges that have two selected vertices are selected.
Many of the convert selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.