Changes the current polygon component selection to any polygon faces that can be selected depending on whether the original selection contains a region.
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 Faces, any selected vertices that indicate a four-sided region are selected as faces. If a region only contains three selected vertices, the corresponding face will not be 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.