What is a Master Unsubscribe?

When a subscriber unsubscribes from all emails sent from an account

About the other options: - When a subscriber unsubscribes from a Publication list via an email unsubscribe link : using the unsubscribe link you will have a list level unsubscribe. -When a subscriber unsubscribes from a list in the Subscription Center: also using Subscription Center, if he unsubscribe from a single list, this is a List Level unsubscribe. - When a subscriber unsubscribes from all emails sent from the Salesforce Marketing Cloud: in this case we will have a Global Unsubscribe.