What is the key characteristic of checked baggage?

Checked baggage is defined by its key characteristic of being under the airline's custody during the flight. This means that once a passenger checks their baggage in at the airport, the responsibility for the baggage shifts to the airline, which transports it in the cargo hold of the aircraft. Passengers do not have access to their checked baggage during the flight, and it is managed by airline personnel.

In the context of air travel, the other options do not accurately reflect the nature of checked baggage. For instance, checked baggage does not stay with the passenger at all times, as passengers separate from their bags after check-in. While emergency supplies may be part of baggage for certain emergencies, they do not represent the broader category of checked baggage, which typically consists of personal belongings. Additionally, checked baggage is not exclusive to crew members; it is utilized by all passengers traveling on the flight, further affirming that the correct answer captures the essential function and responsibility associated with checked baggage for all travelers.