What are the fields used to determine if a contact is new or existing with Salesforce?

Could you tell me what the matching fields are that are used to determine if a contact is new or existing?  Is it all the fields that are being mapped on the form?  Is it the email field?  Also, if in a SF contact record there are multiple email fields, do you look at all of those to compare to the email on the form to decide if the contact is new or existing?

Hello Meg,

Yes, it's the email address field, and the integration picks up the default/primary email address, not the other email addresses fields which are added manually on your Salesforce fields.