There is no stack ranking involved (as far as I know, at least in SW). That's why when an engineer is evaluated as under-performing this is really about their own (manager-perceived) performance vs the expectation, and there is no question about whether they'd be in an exceptional team. Now instead of a formal PIP that we know won't get anywhere, HR isn't shy about letting go dead-weight. My impression is that the default approach from HR when a manager rate someone as under-performing is that we should let them go, unless the manager wants to setup some sort of PIP.