The router always tries the lowest tier first. This only caps how far it may fall back.
Useful for comparing a local model against Google on the same sentence.
Translates the result back through a different engine and scores how much survived. Costs an extra call.
Neural voices where they exist, on-device MMS voices where they do not.
On-device uses MMS. It sounds more robotic but needs no network, and it is the only voice for Assamese, Odia and Maithili.