What does the black dot typically indicate on a flammability diagram?

The black dot on a flammability diagram primarily indicates the limit of combustion (LOC). This point represents the conditions under which a substance can maintain a combustion reaction. Specifically, it marks the threshold at which the fuel-air mixture can sustain combustion without being extinguished. Below this point, the mixture is either too lean or too rich to support continued burning; thus, it cannot sustain a flame.

In a flammability diagram, understanding the LOC is essential for assessing fire hazard and risk in different environments. The other options refer to different aspects of flammability - the flash point indicates when a substance can ignite, while the lower and upper flammability limits denote the concentrations of fuel in air that can support combustion, but do not signify sustained combustion themselves.