Product reuse allows you to quickly create a product by reusing existing products. For example, a product that is called drivetrainA contains three products: engine, transmission, and differential. A new drivetrainB product can be created by reusing the same engine and differential, but with a different transmission. Furthermore, drivetrainA and drivetrainB can be reused in multiple higher level products, such as products that represent an entire vehicle.
The term child product is used to refer to a product that is used by another product, whereas the term top-level product refers to a product that is not used by any other product.
In the image, notice that the topmost blue product has four child products, while the topmost green product has two child products. All the child products belong to the topmost red product. If you were to reuse the topmost green product, you would not get the child products from the topmost blue product. Likewise, if you were to reuse the topmost blue product, you would not get the one child product from the top-level red product.

You can see the same information visually in the Browse Products page product tree.

SuperCar is the top-level product with many child products. Body Shell contains a child product called door. Let's say SuperCar has four doors; instead of creating four products that are called door, you can reuse the same door product three times. Because handle, trim, and window are part of door, they are also reused. Additionally, if all the child products under Body Shell were appropriate for use in a product that is called LuxuryCar, you could reuse Body Shell and all of its child products in LuxuryCar.
When you reuse a product (and any child products) elsewhere in the product tree, a reference is used. Each product has only one instance in the list of products in the tree. You can reference the product in one or more locations. If you want a copy, you must create a product. You can also reuse child products. To reuse child products by creating references, control-select the child products that you want, right-click, and then select Reuse.
The advantage of reuse is that if you must change a child product, you need to do so only once and the change is propagated to all the reused child products. If you must change a child product and want to see how many other products use the child product, right-click over the child product and choose Find Use. The Find Use page shows all the products that use the child product.