The concept behind fixed tokens is relatively straightforward; if an actor is entitled to 100 1% virtual fixed tokens then, at maturity of the IRS pool, they can claim exactly 1 token of the underlying. Note this assumes the term of the IRS pool is precisely one year. On the other hand, 1 virtual variable token gives an actor exposure to the variable cash-flows that can be generated with 1 token worth of deposits in the underlying yield-bearing pool (e.g. aDAI), over the term of the IRS pool.