Publication lists can provide more control over opt-out (unsubscribe) requests in Salesforce Marketing Cloud (SMC). Without a publication list, unsubscribe requests opt-out from all commercial email from your Business Unit in Salesforce Marketing Cloud. With a publication list, an unsubscribe request opts-out from only that publication list. For example, if you choose a publication list when sending your newsletter, opt-out requests affect only that newsletter (rather than your SMC Business Unit).