By default, all HubSpot contacts will sync to Salesforce. However, if you create an inclusion list in HubSpot, only contacts that meet the criteria of your inclusion list will sync from HubSpot to Salesforce.
On June 22, 2017, default inclusion lists were set for all previously existing Salesforce-integrated customers who did not have an inclusion list. The list includes all contacts, except those with a lifecycle stage of Subscriber and will appear in your account as Salesforce inclusion list (default). Accounts created after June 22, 2017 will not have an inclusion list by default and all contacts will sync unless an inclusion list is created for your account. You can learn more about creating inclusion lists here.