TYPO3 v9 has reached its end-of-life September 30th, 2021 and is not maintained by the community anymore. Looking for a stable version? Use the version switch on the top left.
You can order Extended Long Term Support (ELTS) here: TYPO3 ELTS.
auth key is used for configuration of authentication services. However,
only a single and rarely used option is available currently:
Specifies a URL to redirect to after login is performed in the backend login form. This has been used in the past to redirect a backend user to the frontend to use frontend editing.