Trimmed is a mod which adds several client side systems to make vanilla's armor trims viable for mods and datapack authors alike.
External Item Model Overrides [prior to 1.21.4]
Vanilla requires you to overwrite the model you want to add override predicates to, but with Trimmed you can add special json files into models/item/overrides to create many different kinds of model overrides for any item. Trimmed itself adds two types for nbt testing or trim materials.
Client "Tags"
This system works similarly to datapack tags. This allows trim patterns to work with other datapacks which integrate Trimmed.
Client "Maps"
This is a new json system which contains unique pairs. This allows trim materials to work alongside other datapacks with Trimmed.
For more information on how to use Trimmed, please see the wiki.