Tank capacity
Carpet shampooers are generally fitted with two water tanks, one for clean water and one for dirty water. The shampooer pushes out clean water mixed with the detergent, then sucks it back up into the dirty tank. You'll have to keep refilling the clean tank and emptying the dirty one as you go. Having a large tank means you'll have to refill it less often, but will involve moving around a heavier piece of equipment.