By default (using business accounts), Marketing Cloud Account Engagement creates new records as leads in Salesforce. This means that when a prospect is assigned to a user in Marketing Cloud Account Engagement, and there is no matching lead or contact in Salesforce, Marketing Cloud Account Engagement will create a new lead record in Salesforce and sync with it. However, you can also configure Marketing Cloud Account Engagement to create new records as contacts or person accounts in Salesforce, depending on your preference. To do this, you need to enable the option in the Salesforce connector settings in Marketing Cloud Account Engagement, and make sure that the connector user has the appropriate permissions in Salesforce. For more details -> 151617