This features helps prospective Shield customers test Shield through a trial/proof of concept in their production environments, without impacting the broader end user experience. Admins can choose which Box users are actively testing Shield classification-based security controls, and only those users will be able to see and apply new classification labels (created for the purposes of the trial) and see Shield's access policies enforced.
Article with more detail: https://community.box.com/t5/Using-Box-Shield/Testing-Shield-in-Trial-Mode/ta-p/81612