URL query attribute to indicate the wrapper used to render a request.
The wrapper format determines how the HTML is wrapped, for example in a modal dialog.
const WRAPPER_FORMAT = '_wrapper_format';
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https://api.drupal.org/api/drupal/core!lib!Drupal!Core!EventSubscriber!MainContentViewSubscriber.php/constant/MainContentViewSubscriber::WRAPPER_FORMAT/8.1.x