Implements hook_system_info_alter() for the field module. Examines each enabled module that provides a field type by implementing hook_field_info and marks the module "required" if any fields of that type are now in use. A required module cannot be disabled.

• &$info is an object that is updated to show which modules are required
• $file is an object that describes a module or theme
• $type is 'module' or something else (such as 'theme')

Description on api.drupal.org