Marketing Cloud Account Engagement syncs first with contacts in Salesforce. Marketing Cloud Account Engagement and Salesforce work together to keep data aligned between the two platforms. Some of the data that syncs bi-directionally are leads, contacts, and campaigns. Some of the data that syncs one-directionally from Salesforce to Marketing Cloud Account Engagement are accounts, opportunities, and tasks. By default, when Marketing Cloud Account Engagement syncs a prospect, Marketing Cloud Account Engagement looks for contacts with a matching email address as first priority. If it doesn't find any, it will look for a matching lead to sync with. If it doesn't find one of those, either, it will create a lead. This ensures that Marketing Cloud Account Engagement does not create duplicate records in Salesforce. You can also configure Marketing Cloud Account Engagement to create contacts or person accounts instead of leads, depending on your preference. For more details -> 678910