What does the 'export' key mean in a schema definition? The code below is from the message module.  What does the 'export' section mean?

function message_schema() {
  $schema['message_type'] = array(
    'description' =&gt; 'Storage for user-defined message templates.',
    'export' =&gt; array(
      'key' =&gt; 'name',
      'identifier' =&gt; 'message_type',
      'default hook' =&gt; 'message_default_message_type',  // Function hook name.
      'primary key' =&gt; 'name',
      'api' =&gt; array(
        'owner' =&gt; 'message',
        'api' =&gt; 'message',  // Base name for api include files.
        'minimum_version' =&gt; 1,
        'current_version' =&gt; 1,
      ),
      'load callback' =&gt; 'message_type_load',
      'save callback' =&gt; 'message_type_save',
    ),
    'fields' =&gt; array(
      // Although the "name" should be enough as the primary key, the numeric ID
      // is required for the internal use of entity API.
      'id' =&gt; array(
        'type' =&gt; 'serial',
        'not null' =&gt; TRUE,
        'description' =&gt; 'Primary Key: Numeric message type ID.',
      ),
      'name' =&gt; array(
        'description' =&gt; 'The unified identifier for a message type.',
        'type' =&gt; 'varchar',
        'length' =&gt; 255,
        'not null' =&gt; TRUE,
        'default' =&gt; '',
      ),
      'description' =&gt; array(
        'description' =&gt; 'Description for this message.',
        'type' =&gt; 'varchar',
        'length' =&gt; 255,
        'not null' =&gt; TRUE,
        'default' =&gt; '',
      ),
    ),
    'primary key' =&gt; array('id'),
    'unique keys' =&gt; array(
      'name' =&gt; array('name'),
    ),
  );
These properties are used for implementing ctools export/import APIs.

See How to make configuration objects exportable with CTools.