HawkOpen reads delivery failure notices from your connected mailbox and decides by their status code: codes starting with 5 are hard bounces, codes starting with 4 are soft. A notice with no readable code is treated as soft, so that a temporary problem never blocks an address for good.
An address with repeated soft bounces is also set aside. The bounce rate is the share of sent emails that bounced; on the Business plan, the Deliverability Guardian pauses bulk sending when hard bounces get too high.