When you’ve built a product you can stock it, sell it, or use it just like any other component in another product - level after level after level. Creating the product is easy as well. All your parts are held in one database, and cross indexed by type, part number, location, supplier, manufacturer, etc. You can copy parts lists, modify them, add ‘time’ as a costed item, specify alternate parts, and use fractional lengths, volumes etc. The parts lists even have issue numbers.