Regions are two-dimensional enclosed areas that have physical properties such as centroids or centers of mass. Two or more existing regions can be combine into a single, complex region to calculate an area.
A region can be created from an bojects that form closed loops. Loops can be a combinations of lines, polylines, circles, arcs, ellipses, elliptical arcs, aand splices. Regions can be used for, applying hatch and shading. Regions can also be used in analyzing properties, such as area by using command Massprop and List. Regions can as well be used in extracting design information, such as centroid.
There are lot of ways in AutoCAD to create regions,but the easiest way I know of creating regions are by using Boundary command. This command can be found under the Draw pulldown menu. Or at the command prmopt, type Boundary or BO for shortcut. After keying-in Boundary command, a dialogue box will appear, just click the Region or Polyline on Object type, then click Pick points, and the n select the internal point you want to be created as region.