The world of postage stamps has many classifications. Most stamps function as regular, everyday postage on letters and packages, and they have no special designation inscribed upon them. We call these stamps 'regular issues' or 'definitives' and 'commemoratives'.
'Special use' stamps cover services other than those handled by normal postage, and special service inscriptions almost always identify these services. Such classifications or services include semi-postal, airmail, special delivery, airmail special delivery, certified mail, postage due, official use, newspaper and periodicals, parcel post, special handling, postal insurance, military, war tax, and other special categories.
The Scott catalog lists regular issues and commemoratives first, but some stamp catalogs mingle their listings of regular and commemorative issues with some 'special use' stamps, such as semi-postals, airmails, and special deliveries. It is more common in the United States to list regular issues and commemoratives first while placing all other categories—'special use' stamps—separately after the regular issues and commemoratives (hence the expression 'back-of-the-book' stamps, another term for 'special use' stamps).
Catalogs use special prefix letters to designate special use stamps. The Scott catalog uses “B” for semi-postal, “C” for airmail, “E” for special delivery, “PR” for newspapers and periodicals, and so forth. Catalogs including the semi-postals, airmails, and special deliveries in the regular postage sections do not use prefixes for those stamps, but they do include prefixes after the regular postage listings.