During fermentation yeast transforms sugars into alcohol and carbon dioxide. Attenuation is the percentage of sugars that yeast consume during this transformation. A 100% attenuation would result if the final gravity (FG)of 1.000 or less. This is what wine and dry mead makers are looking for. But, it's not the case when making beer. For example, a beer with an original gravity (OG) of 1.052 and a final gravity of 1.013 would have a 75% apparent attenuation. Here's an equation for figuring apparent attenuation using specific gravity:
% Apparent Attenuation = (OG - FG / OG -1) X 100
Attenuation is a function of a yeast cell's metabolism and metabolism is a function of an individual strain's genetic makeup. Since yeast strains differ in their genetic makeup, generally so do their attenuation range.
Knowing the attenuation range of a yeast strain allows the brewer to select a yeast strain for a particular beer style. For a malty Scottish Ale a brewer may choose a yeast strain with a low attenuation, while an altbier may require a yeast strain with a high attenuation.
Each strain has a typical attenuation range and this range is affected by mash temperature, fermentation temperature, pitching rate and flocculation.