All HubSpot contacts will sync to Salesforce by default. However, if you create an inclusion list in HubSpot, only contacts that meet the list criteria will sync to HubSpot.
Members of the inclusion list will only sync with Salesforce when a sync is triggered.
If new Salesforce leads or contacts are set to create HubSpot contacts, a contact will be created in HubSpot. However, further updates to the contact/lead from either HubSpot or Salesforce will not sync to the other platform.