Tuffs are further classified by their depositional environment, such as lacustrine tuff, subaerial tuff, or submarine tuff, or by the mechanism by which the ash was transported, such as fallout tuff or ash flow tuff.
Tuff is only correct when used as a noun that references a certain type of rock. If you are writing about a specific commercial product that includes tuff as part of a proper name, this usage is also acceptable.