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.
The Shrink Selection Region command is incrementally undoable. For example, if you shrink a region multiple times, each command is undoable.
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whenever a polygon component is selected.