In DirectIQ, all contacts are assigned to either a Subscribed or Disabled status.
Subscribed Contacts: email address associated with the contact is considered valid, and it will be used during delivery attempts.
Disabled Contacts: not used during email campaigns, just kept for reference.
Disabled status is automatically assigned under the following conditions:
When a contact unsubscribes from a mailing list.
An email from the contact is returned as invalid.
When a contact marks your message as 'spam'.
You can also Disable contact manually. In order to learn how to do it read Managing Contact Record.