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+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
+