Hi Adam,
It's basically people who have requested to be unsubscribed after being sent an email. It's an ad hoc process at the moment to firstly know who the contact is, and then mark them as non contactable. However it seems some do filter through into campaigns (I'm presuming because someone set the override of unsubscribes when creating it)
An example I have today is someone who was sent a campaign email, asked to not be included in any emails. I would like to make sure if that email address was to ever enter back into the DB it is never picked up by any list created in or outside of Marketo Lead dB. At times we will import SF data but again, I need a process that recognises a particular email and doesn't send to it.