There's no typed restriction on what "transducer" means. Having a second tape in your FSM is enough. e.g., In the formal definition of an FSM, in addition to th

There's no typed restriction on what "transducer" means. Having a second tape in your FSM is enough. e.g., In the formal definition of an FSM, in addition to the input alphabet Σ, you also have an output alphabet Γ. FSTs are commonly seen in morphological analysis (e.g., your paper?) and in those cases it's probably string-to-string, but FSTs have other use cases as well. :-) Lucene, for example, uses a string->int FST as part of its inverted index (where the strings are terms and the ints are ~docids, IIRC).