Bucket capacity (Japanese: 最大魔力; literally: maximum magic power; Romaji: haidai maryoku) represents the amount of magical energy a caster can gather at once. (The name comes from the analogy Hajime Kanzaka used to describe how magic works in the Slayers world.)
A person's bucket capacity determines how powerful spells he can cast, as most spells have a minimal energy requirement: If the caster fails to supply enough, the spell won't work. Supplying more energy than necessary on the other hand increases the spell's power.
The Demon's Blood Talismans have the ability to increase one's bucket capacity 4-5 times.
See also pool capacity.