Ad Hoc
If you choose to run unsubscribes on an Ad Hoc basis you can choose either 
  • a Campaign to bring in unsubscribes from on the data selection tab. You can then run this after each campaign to bring over your unsubscribes in the way you choose.
  • An Audience, this will process unsubscribes over all time.

Scheduled Processes
If you choose to run unsubscribes on a Scheduled basis you can choose either 
  • a Campaign to bring in unsubscribes from on the data selection tab. You can then set this to run on a schedule for that campaign (so you can pick up later unsubscribes), don't forget to delete or deactivate this once it has run it's course. There is no way to set a limit on when a schedule should finish.
  • An Audience, this will process unsubscribes over all time (within Mailchimp's time limit) the first time you run it, than after that it would only bring in unsubscribes since the last time the schedule ran.

Real Time Processes
If you choose to run a realtime process you can only work with Audience unsubscribes, not campaigns so these cannot be tied to a campaign nor can you add campaign specific data such as the name or ID in an action.