Salesforce Integration - Contact Sync

Salesforce Integration Overview

Are new Contacts / Leads created in SF if there is a Contact in CC and no matching Contact or Lead in SF?

Or does the integration only sync 'new' Contacts from SF to CC, and only syncs back matched Contacts the other way?

If the first, please advise the process as the documentation does not.

If the second, I guess I'll need to go with Campaign Monitor.