Back home we use these crystals called 'soul gems' to recharge our enchanted weapons. Different crystals hold different calibers of creatures. Petty to black. Petty souls are lowly, weak creatures such as mudcrabs. Black soul gems are for humans. Whenever you soul-trap something, that soul is sent to the Soul Cairn where it stays until it's consumed.
Now that I think about it, I'm a sort of 'living soul gem' for dragons, but their souls aren't sent to the Cairn when they die because I absorb them into myself.