Permit three values:
- always (default), the other input field is always displayed
- exists, the other input field is only displayed when the setting
          contains value(s) not listed in choices*.
- never, the other input field is never displayed. If the setting
         contains any value(s) not listed in choices they will be
         discarded on saving.
* This means, under normal circumstances the admin will not see
  the other input field.  It will only appear after manual editing
  to enter a non-listed _choice or if an update were to remove a
  previously selected _choice