It allows you to register new items and blocks into the game with a few JSON files. Documentation in the Github repository. Problems also go there, not in the comments.
For data related to the registered blocks/items, you can use the global data packs from cotton:
https://www.curseforge.com/minecraft/mc-mods/cotton
For client resources, use a vanilla resource pack. Modpack makers should set the enabled resource packs in the options.txt file.
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