1. As storage for one tree of categories, e.g.

	Top Category 1
		Subcategory 1-1
		Subcategory 1-2
			subcategory 1-2-1
	Top Category 2
		Subcategory 2-1

2. As a storage of several trees of categories, e.g.

	Model 1
		Top Category 1
			Subcategory 1-1
			Subcategory 1-2
				subcategory 1-2-1
		Top Category 2
			Subcategory 2-1
	Model 2
		Super Category 1
			Super Subcategory 1-1
			Super Subcategory 1-2
				super subcategory 1-2-1
		Super Category 2
			Super Subcategory 2-1