Originally Posted by Bonden3000
i mean its not in the other sets. what do they do?
Mostly, they simply have a larger bucket, with at least 15% more space to provide head space for the krausen. In the US, the typical 5 gallon (19.6L) fermenter actually is closer to 6.5 gallons (25.5L) in actual volume, or more.
Another alternative, more typically used with carboys, is a blow-off tube, which is where you have a plastic tube fitted to the mouth of the carboy, and the other end of the tube in jar or smaller bucket holding water (or a water/sanitizer solution) to let the gas escape as bubbles. That allows the gas and krausen to be forced out of the fermenter without risking contamination or oxidation, and without significant mess. See here
for a typical blow-off setup.