Function answers_through_a_reply 

fn answers_through_a_reply(command: &Command) -> bool

Whether a command reports its outcome through the reply rather than through an update on the subscription.

These are the ones with nothing behind them on the wire: a client that omits reply_to for one of them has no second way to learn what happened. A biometric release’s reply is the blob itself, so it is one of them by construction.