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