What is the Salesforce Security Owner?

The Salesforce security owner, found on the sync settings tab within the Campaign Monitor Client table, is used to set the record owner for the records we create. Typically the Salesforce sync user is the same as the Salesforce Security owner. When the sync runs we set the record owner of the records we create using the Salesforce Security Owner, for example when we create a subscriber list membership record.

This is useful if you want to use Salesforce sharing to limit access to records.  More on restricting access with the Salesforce security owner.