From the itch.io page for 24XX:
24XX is the free version of the rules used by the 2400 series of RPGs, released under a Creative Commons Attribution 4.0 license...
24XX games prioritize description and narrative positioning over detailed rules processes, taking the highest die rolled over mental arithmetic, and polyhedral dice over only one shape of die (because funny-shaped things are fun to roll). While 2400 is a sci-fi series, you're welcome to use these rules for whatever setting you like. And if you use any of this material in your own games and hacks, please feel free to link to your games here on the discussion board!