hook_notifications_object_ENTITY_TYPE()

hook_notifications_object_ENTITY_TYPE() returns, among other things, the available subscriptions for a particular entity. These will make their way into the subscriptions block.

For example, if you are viewing a node, then modules implementing hook_notifications_object_node() can specify which subscriptions will be available for users to subscribe to in the subscription block when viewing that node. Here's an example from og_notifications:

/**
 * Implements of hook_notifications_object_node()
 */
function og_notifications_notifications_object_node($op, $node, $account = NULL) {
  switch ($op) {
    //...
    case 'subscriptions':
      // Return available subscription options for this node
      $options = array();
      if (array_keys(og_get_all_group_content_entity()) && notifications_content_type_enabled($node->type, 'group_content')) {
        $options[] = notifications_subscription('group_content')
          ->add_field('node:gid', $node)
          ->set_group($node)
          ->set_name(t('All posts in this group.'));
      }
      return $options;
      break;
  }
}

This code tests for the $op being equal to 'subscriptions', and then instantiates the relevant subscriptions for any node that is a group node.