If a fuel is below its upper flammable limit, how is it classified concerning burning?

When a fuel is below its upper flammable limit, it is classified as "too lean to burn." This means that the concentration of fuel in the air is not sufficient for combustion to occur. In such a scenario, there is not enough fuel present to create the necessary conditions for a sustained fire. For combustion to take place, a specific ratio of fuel to air is required, which is referred to as the flammability range. The upper flammable limit indicates the maximum concentration of fuel that can be present in a mixture with air for ignition to occur. When the fuel concentration dips below this limit, the mixture becomes too lean, meaning there is an inadequate amount of fuel relative to the air to support combustion.