Applications
Guide numbers are especially useful in dealing with manual flash units when calculating the aperture for a particular exposure or the maximum flash range for a given aperture. In practice, it's normal to know the guide number and the distance and need the aperture, so the equation is rearranged to give:
f-number = GN/distance
Some flash units have a dial on the back (in fact, a simple form of slide rule) or a chart to help in calculating the desired settings.
The aperture given by the guide number is only correct for certain locations, generally indoors where a reasonable amount of "stray" flash light will be reflected from surrounding walls onto the subject. The effective GN is slightly lower outdoors as any light not falling directly on the subject from the flash will be lost.
Read more about this topic: Guide Number