Any cylinder containing a gas other than air must be:

A cylinder containing a gas other than air must be clearly labeled to ensure safety and proper handling. Labeling allows divers and other personnel to easily identify the contents of the cylinder, which is crucial for safe practices during gas mixing, filling, and usage. Accurate labeling helps prevent dangerous situations that could arise from using the wrong gas, as different gases have distinct properties, such as varying oxygen levels, pressure requirements, and potential hazards.

While other options may describe best practices or standards for handling cylinders, the primary reason labeling is essential is that it directly communicates information about the gas within, contributing to the overall safety of both the users and the environment. For instance, oxygen-rich environments can pose increased risks of combustion, so being aware of the cylinder's contents is vital.