The Whole Sort of General Mish Mash (WSOGMM) is mentioned in Mostly Harmless. The Guide describes the WSOGMM as a way of understanding the sum of parallel universes in creation (in this context, creation is best described to a casual observer as spacetime). The theory of WSOGMM states that parallel universes are not parallel as such but rather each universe is simply a quantum physical perception of the same construct, which is the WSOGMM.
The last version of the Guide gives a description of the Mish Mash to Arthur Dent's daughter Random. The guide explains that individuals in her universe move freely in the first three dimensions, which they know as space, travel in a straight line in a fourth (Time) and are rooted to one place in a fifth, which is described as the first fundamental of probability.
The fifth dimension (and, it is implied, every other dimension up to the twenty-second) is actually an axis of probability that makes up the Mish Mash. Any multiverse in creation is actually a location in the fifth dimension. To an observer, a certain universe functions alone, but it's proven that if something is introduced simultaneously into every dimension (such as the bird version of The Guide was as a marketing ploy), an action on that object can affect the entire Mish Mash. In the final chapter of the novel Mostly Harmless, when Earth is destroyed by the Vogons and the bird dies with it, the Earth is destroyed in all other universes.finally removed from the WSOGMM.