Here is the list of the values and a brief description:

Unknown - This is the default value.  It usually indicates that the Constituent has not made a sustaining gift.
Active Sustaining Donor - The Constituent has an Active sustaining gift.
Formerly a Sustaining Donor - This field is deprecated.
Postponed Sustaining Donor - One or more payments on a sustaining has been skipped.
Completed Sustaining Donor - The Constituent has completed a sustaining gift.
Terminated Due to Payment Failures - The Constituent's sustaining gift has been cancelled because of too many payment failures.
Terminated by User - The Constituent has cancelled their sustaining gift.
Terminated by Admin - The Constituent has a sustaining gift that has been cancelled by an administrator.
Lapsed Sustaining Donor - The Constituent has a sustaining gift with transactions that are failing, but still being attempted.
Partially Lapsed Sustaining Donor - The Constituent has as least one active sustaining gift and at least one lapsed sustaining gift.