I'm not sure if this is obvious, if what it means in practice is a metric that may be the wrong metric, but which is visible, being replaced by a metric that is

I'm not sure if this is obvious, if what it means in practice is a metric that may be the wrong metric, but which is visible, being replaced by a metric that is invisible. At least in the first case we can reason about the metric and why optimizing for it might go wrong.