Which category of gas is not considered "non-flammable"?

The category of gas that is not considered "non-flammable" includes flammable gases such as propane or acetylene. These gases are specifically designed to ignite easily and can sustain a flame, which distinguishes them from non-flammable gases. Non-flammable gases, by definition, do not ignite or support combustion under normal conditions.

Inert gases like argon and non-reactive gases such as nitrogen are categorized as non-flammable because they do not undergo combustion or contribute to fire hazards. Non-flammable gases like helium also fall into this category, as they are safe to use in environments where fire hazards must be minimized. Thus, flammable gases represent a clear departure from the characteristics associated with non-flammable gases.