docs: def classify_identifier(did):
      """Return a text fragment classifying the ``did``
  
      Return <UNKNOWN> if the DID could not be classified. This should not normally happen
      and may indicate that the DID was orphaned in the database.
  
      """
      if _is_unused_did(did):
          return 'unused on this Member Node'
      elif is_sid(did):
          return 'a Series ID (SID) of a revision chain'
      elif is_local_replica(did):
          return 'a Persistent ID (PID) of a local replica'
      elif is_unprocessed_local_replica(did):
          return (
              'a Persistent ID (PID) of an accepted but not yet processed local replica'
          )
      elif is_archived(did):
          return 'a Persistent ID (PID) of a previously archived local object'
      elif is_obsoleted(did):
          return 'a Persistent ID (PID) of a previously updated (obsoleted) local object'
      elif is_resource_map_db(did):
          return 'a Persistent ID (PID) of a local resource map'
      elif is_existing_object(did):
          return 'a Persistent ID (PID) of an existing local object'
      elif is_revision_chain_placeholder(did):
          return (
              'a Persistent ID (PID) of a remote or non-existing revision of a local '
              'replica'
          )
      elif is_resource_map_member(did):
          return (
              'a Persistent ID (PID) of a remote or non-existing object aggregated in '
              'a local Resource Map'
          )
      logger.warning('Unable to classify known identifier. did="{}"'.format(did))
      return '<UNKNOWN>'