By default, Marketing Cloud Account Engagement writes to two objects in Salesforce: contact records and lead records. These are the two objects that store information about individual prospects in Salesforce. Marketing Cloud Account Engagement syncs with these objects based on the email address field, which is the unique identifier for prospects. Marketing Cloud Account Engagement can create new contact or lead records in Salesforce, or update existing ones, depending on the sync settings and rules. Marketing Cloud Account Engagement does not write to case records, opportunity records, or account records by default, but it can read from them and use their information for segmentation and reporting purposes. Reference Marketing Cloud Account Engagement and Salesforce Sync Behavior