Some areas may never be approved for hot work, and these are called nonpermissible areas. These may be areas where sprinklers are impaired, or where an explosive atmosphere could develop due to combustible dust or flammable vapors.
Permissible areas, where hot work is allowed to be performed, are further divided into designated areas and permit-required areas. A designated area is of noncombustible or fire-resistive construction, and it is maintained to be fire-safe. An example is a maintenance shop. A permit-required area is made fire-safe by removing or protecting combustibles from ignition sources. See the clip for more details.
Requirements for permissible and nonpermissible areas can be found in sections 5.2 and 5.3 of NFPA 51B (2014), which can be viewed for free on the Doc Info page.