What is Soft bounce?
A temporary delivery failure — full mailbox, greylisting, a policy or reputation rejection — that can be retried later.
Soft bounces cover everything that isn't "this address does not exist": a full inbox, a temporary server error, greylisting (deliberately deferring a first attempt), or a 5xx rejection on reputation or policy grounds. Treating these as permanent throws away good addresses; treating a reputation rejection as routine hides a domain problem. The useful split is bad-address vs. policy-block, because the second one is about you, not the list.