You can organize items into groups or sub-kits. Or, also parts can have parts. I don't know anything about banjos but a restaurant example is a dinner plate has a part for Sides. The item Sides can have as parts Salad, Soup or Fries. And the Salad item could have parts of toppings and dressings.... So, you can make logical groups and each item in the group can have sub-groups. BTW - a restaurant would not be using parts/kits. They would be using Add-ons which give options but they both work in a similar ways.