Shrinks the region of the currently selected components inwards in all directions from the currently selected component on the polygonal mesh. The characteristics of the reduced selection region/boundary is dependent on the components in the original selection.

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 > Shrink Selection Region, the eight vertices that form the boundary on the selection become unselected on the mesh, leaving a single vertex selected. If you continue to shrink the selection, eventually nothing is selected.