What does the plus sign (+) indicate in the tank marking 9X94+?

In the context of tank markings, the plus sign (+) indicates that the tank is approved to be filled to a higher pressure than its standard service pressure. This means that it can safely be overfilled by 10% beyond its designated fill pressure. This capability is crucial for divers and filling stations as it allows for a greater volume of gas to be packed into the cylinder, which can be beneficial when planning for dives that require more air supply.

Understanding this marking is important for safety and managing air supply during dives. The other options relate to specific characteristics of tanks, such as altitude usage or temperature ratings, which are not represented by the plus sign in tank markings. Therefore, while those aspects are essential for divers to consider, they do not pertain to the meaning of the plus sign in this scenario.