One thing I'm not clear about: in the version that knows the 2315-word list, is it allowed to only guess words from the 2315, or can it also guess words outside

One thing I'm not clear about: in the version that knows the 2315-word list, is it allowed to only guess words from the 2315, or can it also guess words outside of it, but in the 12972-word list? It seems to me like with more flexibility in what words to guess, it might be able to eliminate words faster and build a more-optimal tree.