Changes the current component selection to select only the components that form a boundary around the current component selection. The previously selected interior components that are not part of the boundary are unselected.
For example, if you have a polygon mesh comprised of four-sided polygons (quads) with nine interior vertices selected, in the shape of a square grid, and then choose Select > Select Selection Region, the eight vertices that form the boundary on the selection remain selected on the mesh. The interior vertex is unselected.
Many of the polygon selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.