On each page the underlying rules for the universal cellular automaton are exactly the same. But on the first page, the initial conditions are set up so as to make the universal cellular automaton emulate rule 254, while on the second page they are set up to make it emulate rule 90, and on the third page rule 30.
The pages that follow [649, 650, 651] show how this works. The basic idea is that a block of 20 cells in the universal cellular automaton is used to represent each single cell in the cellular automaton that is being emulated. And within this block of 20 cells is encoded both a specification of the current color of the cell that is being represented, as well as the rule by which that color is to be updated.