This post can be summed up in one screenshot:

That’ll qualify people who clicked the all-too-tempting Unsubscribe button in Gmail, Yahoo!, etc. instead of visiting your preference center — or preference centers, if you have a multi-brand setup.

The constraint Form Fields [is empty] means a Marketo form wasn’t the source, ergo the List-Unsubscribe header was the source.

(Interestingly, you can’t constrain on the Activity Log Details, though they tell you “lead unsubscribed from list unsubscribe.”)

In the flow, you might take partition-specific action, such as setting the custom field Unsubscribed (B2B) to true and flipping the system Unsubscribed back to false so Durable Unsubscribe doesn’t confuse things. (Of course I’m speaking of environments where brands are legally distinct.)