Specialty bus plugs are used when ordinary bus plugs such as fusible switches or ordinary circuit breakers aren't appropriate. One example of a specialty bus plug is the special construction circuit breaker designed for use on a boat or ship. In these cases, ordinary circuit breakers can't be used. One good example; marine circuit breakers must not have components made of aluminum and must be calibrated for specific temperatures. They are built to withstand the wear and tear of maritime use, including changes in humidity and other factors.
Specialty bus plugs can also include low-voltage circuit breakers or other types made differently than typical "industry standard" bus plugs. It's very important to use the right bus plug for the right needs. In the case of marine-approved circuit breakers, resistance to humidity and corrosion from sea salt is an important factor--just one example of how some bus plugs are tailored for a specific purpose. They should only be used for that specific purpose unless the owner's manual specifies other approved uses.