You can set address books to be private or public (newly created address book's are private by default).
A private address book won't be visible to your contacts when making use of your preference centre, which means they won't be able to sign up or unsubscribe themselves from it.
A public address book, however, is visible in your preference centre. This allows contacts to choose which of your communications they wish to unsubscribe from (known as opting down as opposed to opting out), helping to avoid mass unsubscription from everything you send.
Remember – setting an address book's status to public means your contacts will see the name of it, so make sure the name is sensible and presentable!
Setting an address book as private or public
You can set an address book as private by going to your address book list under Contacts > My contacts. Click on the Properties icon alongside the address book you'd like to change.
You will be taken to the 'Edit address book' screen. Underneath the 'Description' field is the 'Visibility' section, where you can select whether the address book is public or private.
Click on the radio button next to 'Private (do not show in preference centre)' or 'Public (show in preference centre) and then click Save.
Changing an address book's visibility to private will immediately remove it from being shown in your preference centre.
Changing it to public will immediately make the address book visible in your preference centre and will allow contacts to join, or unsubscribe from, the address book.