To sync specific contacts from Sales Cloud to Marketing Cloud without syncing all contacts, particularly when not all contacts need email communications, the admin should use a Boolean field to filter the contacts. This can be configured in the Contact Builder under Data Sources. A Boolean field allows for a simple true/false condition that can be set in Sales Cloud and used to control which records are synchronized based on their relevance for email communication. This method ensures that only contacts marked true (or false, depending on the setup) are brought into Marketing Cloud, efficiently managing the sync and keeping irrelevant contacts out of the system.