Hi Ken. The native sync follows this protocol when updating or creating records in Dynamics. Let's use the scenario of someone filling out a web form on your site. The transaction goes into Marketo first of course and then needs to be passed to a sales rep in Dynamics. Here's what Marketo does.
1) It looks in the contacts object in Dynamics for a matching contact. It uses email as the unique identifier. If there is a match, it updates the contact.
2) If there is no matching contact it looks for an existing lead, again using email as the unique identifier. If it finds a matching lead, it updates that record.
3) If there is no matching lead, it creates a new lead in Dynamics.