If one is accustomed to thinking of the raw Quintessence as a pool, then think of Tass as Quintessence temporarily frozen like ice, floating on the surface of this larger sea. In time, this Tass will "melt," eventually flowing back into the infinite pool of Quintessence that first spawned it. Until it does however, Tass may be used by mages to lend power to their magic and to fuel their mystical Talismans.
Tass collects in certain Nodes where energies gather in physical objects. Tass is sometimes affected by the form in which it manifests, unlike the pure Quintessence from which it springs. Quintessence stored in the waters of a holy spring may lend itself to healing, while that stored in the mushrooms of a faerie ring may cause the magic powered by them to have unpredictable side effects. A mage should be aware of this Resonance and use this resource accordingly. Tass also infuses the bodies of magical creatures -- werewolves, faeries, and the like -- and it may be harvested from them. Needless to say, such creatures are less than happy to sacrifice themselves for a mage's convenience. More than a few mages have paid the ultimate price for such presumption.
Tass can also generated in certain periapts.