A Contact is a person associated with an Account while a Lead is a prospective client with whom a salesperson is developing a relationship.
Salesforce has a very specific workflow that governs the conversion of a Lead to a Contact. When this conversion occurs, Informz captures the information and updates the subscriber record's ID to use the Contact ID rather than the Lead ID. Additionally, after a conversion, the converted Leads may remain in any Salesforce Lead Reports where they previously resided. However, they are always excluded during a sync with Informz and do not appear in Salesforce Lead target groups.
When a Lead is converted to a Contact, Informz retains all mailing history.