This is a new mode where you fight certain enemies under certain restrictions in an effort to get a new high score and optain items.
Persona Central has a more in-depth breakdown of this particular demo.
Various themes for the different Challenge Battle segments are provided by the Velvet Room twins Caroline & Justine, with the only theme available at the TGS 2019 demo being "Trial." This theme revolves around the fundamentals of combat, which are hitting enemy weak points, support, and Baton Touch. The playable characters available were Joker, Ann, Yusuke, and Morgana.
Point bonuses will be given by defeating enemies within a specified amount of turns or defeating enemies with specific characters. Some examples of the points allocated in the demo are "+15,000 points if the enemies are defeated within 28 actions," "4x score bonus for using nuclear attacks," or "7x score bonus by defeating enemies with Yusuke."
More at the link
https://personacentral.com/persona-5-royal-tgs-2019-demo/