What does the 'S' indicate in the POB status check?

In the context of a POB (Portable Oxygen Bottle) status check, the 'S' indicates the sealed status of the oxygen mask. This designation is crucial because it ensures that the mask has not been tampered with or compromised before use. A sealed mask guarantees that the integrity of the oxygen supply is maintained, which is vital during emergencies when reliable access to oxygen may be required.

Maintaining the mask in a sealed state ensures that it remains free from contaminants and is ready for immediate deployment in case of an emergency, thus enhancing the safety of both crew members and passengers. The importance of regular checks on the sealed status cannot be overstated, as it directly affects the effectiveness of the oxygen system when it is needed the most.