There are a lot of "items" that are not real, actual items. These items are referred to as Abstract Items (or Abstractions).

These Abstract Items will often appear in a recipe, where any number of items (of all similar "type") can be used to execute the recipe.

E.g., if you are asked for a "Metal Binding" you could use an "Iron Binding" or a "Copper Binding" (or many others). However, you will never find an actual item in the game named a "Metal Binding."

Below are (most) all of the known Abstractions and corresponding items.