Creating regions  

You can convert a closed entity into a two-dimensional region. After you create a region, you can modify it using the various three-dimensional tools. For example, you can create a region from a square, and then extrude the square to create a three-dimensional cube.

You can create regions from closed entities, such as polylines, polygons, circles, ellipses, closed splines, and donuts. You can also select open entities such as lines and arcs, if they have common endpoints and they form a closed area.      

Creating regions typically has no visible effect on a drawing. However, if the original entity had a width or lineweight, that information is lost when you create the region.    

To create a region

   1   Do one of the following to choose Region ():

    On the ribbon, choose Draw 3D > Region (in Draw 3D Solids).

    On the menu, choose Draw > 3D Solids > Region.

    On the Draw 3D Solids toolbar, click the Region tool.

    Type region and then press Enter.

   2   Select the entities to create the region.

   3   Press Enter.

The command bar displays a message that describes how many regions were created.

The Boundary command can also be used to create regions.
For more details, see Defining boundaries.