What must be true for a Salesforce Opportunity to sync to Marketing Cloud Account Engagement?

For a Salesforce Opportunity to sync to Marketing Cloud Account Engagement, the Opportunity must have a Contact Role that is syncing to a prospect in Marketing Cloud Account Engagement. This means that the Contact Role must be associated with a Contact record that has a Marketing Cloud Account Engagement prospect record. The Opportunity record type, source, or creator are not relevant for the sync1