Volumes of CO2 is the standard unit for describing how carbonated a beer is — literally, how many litres of CO2 gas (measured at standard temperature and pressure) are dissolved in each litre of beer. Most beer styles fall somewhere between roughly 1.5 and 3.0 volumes, with real variation across styles that's worth targeting deliberately rather than defaulting to one number for everything.
Different styles genuinely call for different targets
English cask ales traditionally sit at the low end, around 1.5-2.0 volumes, matching their soft, low-carbonation cask-conditioning tradition; most American ales and lagers land in a middle range around 2.2-2.7; German wheat beers and Belgian styles frequently push considerably higher, sometimes reaching 3.5-4.0 volumes or more for the most effervescent examples. Using a single generic carbonation target across every style ignores a real, checkable stylistic difference.
The number every priming and force-carbonation calculation is built around
Volumes of CO2 is the actual target figure both the priming sugar calculator and any force-carbonation pressure chart are solving for — everything else (sugar type, batch volume, temperature, keg pressure) is just the specific mechanism used to reach a chosen volumes-of-CO2 target, not a separate goal in its own right, and stating that number explicitly is the first step in either calculation.