What do SWD and HID rating signify for circuit breakers?

Circuit breakers used to switch 120V or 277V fluorescent lighting circuits must be listed and marked "SWD" or "HID." Circuit breakers used to switch high-intensity discharge lighting circuits must be listed and marked "HID." UL 489 permits "HID" breakers to be rated up to 50A, whereas an "SWD" breaker is rated up to 20A.