Once pyrolysis has filled the air above a surface with flammable vapour, that mixture can start burning in two distinct ways. In piloted ignition an external flame, spark or glowing ember lights the vapour, and the surface temperature needed is comparatively low, around 300 to 380 degrees Celsius for common woods. In autoignition, also called spontaneous ignition, no pilot is present and the gas mixture must be hot enough to react on its own, which requires a much hotter surface, typically above 500 to 600 degrees Celsius. The practical rule is that the presence of a pilot lowers the ignition threshold substantially, so an ember landing in dry grass starts a fire under conditions where radiant heat alone would not. The confusion this resolves is the idea that there is a single ignition temperature for a material; the number quoted always depends on whether a pilot was present and on how the sample was heated. After this Concept you can say which of the two ignition routes applies to a given situation and which threshold it must clear.
Flash point is the lowest temperature at which a fuel emits enough flammable vapor to briefly ignite when exposed to a pilot flame, but not enough to sustain combustion, while fire point is the lowes…