Mailchimp to Sugar
If someone unsubscribes in Mailchimp: In Sugar, the contact/target/lead that matches the email address that unsubscribed is removed from Target List, and the email address is marked opted out.
If an email is marked cleaned in Mailchimp (due to hard bounces or multiple soft bounces): In Sugar, the contact/target/lead that matches the email address of the hard bounce is removed from Target List, and email address is marked invalid.
Sugar to Mailchimp
If you remove a contact/target/lead from a Sugar Target List: In Mailchimp, the person is unsubscribed from the Mailchimp audience.
If you set a contact/target/lead's Email as opted out in Sugar: In Mailchimp, the person is unsubscribed from the Mailchimp audience.
Global Opt Outs
Global opt outs describe how unsubscribes are handled when you have multiple records with the same email address. This means that if someone unsubscribes on the Mailchimp side, the contacts/targets/leads in Sugar with a matching email address will be opted out in Sugar and removed from any synced Sugar Target Lists. If the person was on multiple target lists synced with Mailchimp they will be removed from all of those target lists as well.
Global Opt Outs Disable Options
There are two other options for how to handle global opt outs. You can find more information on customizing this here: