Complex sentence: Rule 90 and Rule 102 are "additive cellular automata": if two initial states are combined by computing the exclusive or of each their states, then their subsequent configurations will be combined in the same way.
Split into simpler sentences: Rule 90 and Rule 102 are called "additive cellular automata". This means that, if two initial states are combined by computing the exclusive or of each their states, then their subsequent configurations will be combined in the same way.