Dell advertises a maximum brightness of 500 nits across all current XPS 13 9300 SKUs regardless of display resolution. Our own tests, however, show a maximum brightness of about 600 nits when under certain conditions. Here's how to enable that "extra" brightness.
https://www.notebookcheck.net/The-XPS-13-9300-display-can-be-20-percent-brighter-than-what-Dell-is-letting-on-but-you-ll-need-a-few-clicks-to-enable-it.458953.0.html