From v2.17 onwards there is a change in the checkout process to allow Leads or Contacts to be created if there is no match. By default, a Contact is created.

On Event Checkout, to enable management of Leads with create or update, set the Event Settings as follows:

Create/Update Event Setting

In Event Settings record define

• Registration Entity Type = Lead
• Update the Event Settings lookup on Event to reference the settings

Matching logic order:

1. Attempt to match on existing Contacts as defined by Salesforce Contact Matching Rules (which starts with identical email matching)
2. If there is a match, set the Attendee to the matching Contact.
3. If there is no match, attempt to match on existing Leads by Salesforce Lead Matching Rules.
4. If there is a match, set the lookup on Attendee to the matching Lead.
5. If there is no match, a new Lead will be created and associated with Attendee.

When a Lead is converted, the Contact Id is automatically updated on the associated Attendee record (the lookup from Lead to Contact is carried over automatically).