Imagine three different scenarios in Salesforce: (not using the Convert Person flow step in Marketo)
Converting a Lead into a new contact and new account
Converting a Lead into a new contact in an existing account
Converting a Lead to an existing contact in an existing account (this works identical to merging)
In all three cases you end up with 1 contact and no leads in Salesforce and 1 contact and no people in Marketo.
In Marketo, the record will now have an SFDC Type = Contact.
When converting in Salesforce, make sure your lead custom fields are mapped well. You don’t want to lose any data.
You can trigger and filter using: “Lead is Converted” and “Lead was Converted.”