There are four recognised stages in the development of any fire: Ignition, Spread and Growth, Flash point into a fully developed fire and Eventual Decay. During the early stages, when a fire is still becoming established, the important factors are: Ignitability, Spread of Flame, Smoke Production and Flaming droplets.
For a material to be classed as either Non-combustible or of Limited Combustibility it must achieve Euroclass A1 or A2 in reaction to fire testing. Any material rated Class B or lower is therefore classed as Combustible, albeit to varying degrees.
Whilst it is not possible to enhance any organic materials to a Class A rating, flame retardant treated wood-based materials enhance safety, add value and are fit for purpose for many applications.