How do you know it’s in 3.4? I’m not aware of it, and the fact that there are files with that name doesn’t necessarily mean it’s working or ready for public use.
Generally speaking, one activates alternate scoring methods by setting their scorefunction weights to a nonzero number. The simplest way to add a term for fixbb is to copy the default score12 scorefunction (database/scoring/weights/), add the terms you want, and then pass the new weights file to -score:weights.
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