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.
Tip:
Many of the convert selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.