Full Idea
Machine functionalism requires a mental state to be a physical realisation of a Turing machine; causal-theoretical functionalism only requires that there be appropriate "internal states".
Gist of Idea
Machine functionalism requires a Turing machine, causal-theoretical version doesn't
Source
Jaegwon Kim (Philosophy of Mind [1996], p.112)
Book Reference
Kim,Jaegwon: 'Philosophy of Mind' [Westview 1998], p.112
A Reaction
Searle's objection to the Turing machine version seems good - that such a machine has an implicit notion of a user/interpreter, which is absent from this theory of mind.