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items_packs and resource_pack

What are these folders?

contents

Is a folder which contains a set of folders. Each one contains separated packs which contain configurations and models/textures/sounds...
You can see that each folder name in contents is the namespace of the assets it contains.

configs

Each folder inside contents contains a sub-folder called configs.\ This is a folder which contains a set of folders and/or files. It contains organized .yml files which have items behavior, settings, enchant, recipe, loots... everything that is related to items creation, recipes and behaviors.
Namespace is also set inside the .yml file itself, don't forget to set it in the top part:
info:
namespace: example

resourcepack

You can see this folder and configs as a very similar thing, but resourcepack folder contains the "graphical" part of all items (and also sounds, GUIs...). This folder is automatically compressed inside ItemsAdder/output/generated.zip, the one that will be sent to players.

Important

Remember to always maintain order in contents sub-folders, don't paste things randomly, don't create too many subfolders, don't leave unused textures/models or it will be difficult to find out errors and mistakes.