The Guide Number is another way of stating BCPS. The Guide number tells you what aperture setting you will need on the camera for a given light to subject distance at a given film speed. For example, if you are using an ISO 100 film, you would use the ISO 100 GN and divide this number by the distance from light to subject to find the correct lens aperture. If, for example, the ISO 100 GN for your flash were 110, and your subject were 10 feet from the flash, you would use an f11 aperture. By looking at an appropriate chart, you could also find that an ISO 100 GN of 110 is simply another way of stating a rating of 2800 BCPS. Like the BCPS rating, the Guide Number only has relevance when the flash is used direct.