47 CFR 15.5 prohibits intentional damped wave transmissions such as spark-gap transmitters which were common before the 1920s but occupy a needlessly wide range of frequencies.
Subpart B deals with unintentional radiators—devices for which the purpose is not to produce radio waves, but which do anyway, such as computers.
Subpart C deals with devices that are specifically designed to produce coherent radio waves, such as small transmitters.
On the standard AM broadcast band, under 15.219, transmission power is limited by 100 milliwatts of DC input power to the final RF stage (with restrictions on size, height of, and type of antenna), or alternatively, under 15.221, if the AM transmission originates on the campus of an educational institution, the transmission can theoretically be any power so long as it does not exceed the field strength limits stated in 15.209 at the perimeter of the campus, 24,000/fkHz μV/m.
Frequently encountered types of "Part 15" transmitters include: Electronic equipment from computers to intentional transmitters can produce unwanted radio signals and are subject to FCC regulation.
For digital devices including computers and peripherals, FCC Class B is the more stringent standard, applying to equipment marketed for use in the home, even if it could be used elsewhere.
Class A is a looser standard for equipment intended only for business, industrial and commercial settings.