Final answer:
With the SEAT BELTS switch on AUTO, cabin seat belt lights will illuminate based on pre-set conditions such as altitude changes or turbulence to remind passengers to fasten their seat belts for safety.
Step-by-step explanation:
When the SEAT BELTS switch is selected to AUTO on an aircraft, the seat belt lights in the cabin will typically illuminate under certain conditions which are predefined by the aircraft's safety system. These conditions commonly include instances when the aircraft reaches a specific altitude during ascent or descent, when the aircraft experiences turbulence, or when the pilot manually activates the seat belt sign. By illuminating the seat belt lights, passengers are alerted to fasten their seat belts, ensuring safety during critical flight phases or unexpected situations.