Surface region derives a second surface from part of an original surface. A subregion of a surface is sometimes necessary for an auxiliary surface to use for surface/surface intersection, filleting, milling, and so on.
When you extract a region, you are getting a band of either columns or rows and this is an exact operation within the limits of the source surface.
Surface region is an exact process, however, if the source material is approximate, the resulting region can be no more precise than the source material.