Use millboard any place which will be subject to extreme heat. Most of it is capable of withstanding temperatures of 2,000 degrees Fahrenheit or more. It can block heat, withstand heat of substances such as molten metal, and can seal off extremely hot areas. It is easy to cut, punch with a die cutter or even mold into shapes --- although it is used around furnaces and fireplaces in its rigid form.
Choose a millboard specifically for your project. Some is made with kaolin, a silica material, or similar form of clay. Some types use a form of cellulose, others ceramic materials. Some are composed of cement fibers. The temperature rating will vary with the fiber content and some millboards use a combination of fiber types. The composition also will affect the ease of cutting and installation. Asbestos millboard is still made and used in some parts of the world and has some special industrial applications in America.
Cut your millboard with a saw, although many special situations use a die-cut, in which a shape is stamped out of the millboard; this would be done to line a rotary kiln or industrial boiler or to make gaskets. If you are using a cement-based millboard for exterior wall cladding, you will install it much like wood siding. You can use drywall screws in interior walls or ceilings.