“Tuff” is a generic term for a variety of materials. The only stipulation is that the materials must be ejecta from a volcanic explosion. It can be made up of a variety of materials and can comprise shards ranging in size from dust to boulders.
It is often thickest near the volcanic vent and thins off as you get further away from the volcano. The thickest is on the vent’s downwind side or on the side where the explosion was directed.
Their deposits can be hundreds of meters thick and have eruptive volumes in the millions of cubic miles. This massive thickness can result from a single eruptive blast or, more typically, many surges of a single eruption – or eruptions separated by lengthy periods of time.