Processing
Processing is used to combine 2 or more items to make things that can't otherwise be obtained.
you run the /mix command without giving a recipe it will tell you the things you can currently make and give you a link to all of the processing recipes.
In order to combine the items to create the processed item all you have to do is /mix {target item}
If you want to find the recipe for a specific processed item you can do /recipe {item}. This will tell you what you need in order to create that specific item.
Processing warning
Processing is not a complete system yet, and is still being worked on.