Fliers are bits of burning debris that are detached from the fire front and are carried downwind in its path. Whether they will set new fires depends obviously on their temperature when and where they land and the ignitability or inflammability of the surface on which they land. How far they travel depends upon the speed of the wind, their weight and volume and the height at which they form. In our grasslands the last of these variables is rarely more than two metres but many fliers are light in weight and may carry many metres, even hundreds of metres in advance of the fire-front. They are usually small, not dense and consequently go out quite quickly after landing so that unless there is something easily ignitable at the landing area ,no secondary fire ensues.
Fallers are different.
They comprise burning debris that simply falls in place. In grass, scrub and forest fires the direction of fall is usually with the wind. Unlike fliers, fallers may be large in volume, burning for many hours, and their combustion temperatures are usually higher than for fliers. To protect against fallers, a fire-break must be at the very least as wide as the adjoining vegetation is tall in fact a bit more for fallers which usually bounce and break up as they land. This is because some of the energy in the advancing fire-front dries out the vegetation in front of it.