What are "Replacement Only" Circuit Breakers?

Some circuit breakers made for home use may be labeled "for replacement use only" and "not for CTL assemblies". It may be rare to see this type of labeling on modern Westinghouse circuit breakers or other brands, but it does cause some confusion when it is encountered for the first time. An example of a CTL assembly is a lighting/appliance "load center". These are designed to accept only a full-sized circuit breaker and should not have half-size or mini-circuit breakers placed into them. A circuit breaker made in a smaller size than what is required in the appliance load center may be labelled "not for CTL assemblies" since installation into an appliance load center would be a potential hazard.