When generating mailing labels, CiviCRM users have the option of choosing the address to be used (e.g. you can select "Primary," "Home" or "Work") depending on where you want to reach your contacts.
CiviMail, however, does not provide this option. Instead, CiviMail automatically chooses the address for each contact - a process we can only control on a contact-by-contact basis by setting the "Bulk Mail" or "Primary" flags for each individual contact.