Both documents are necessary because they are used to describe the product in the two major ways you can describe anything: qualitatively and quantitatively. The DID is the document in which the writer can briefly describe the essential features of the product so that the reader can have a more intuitive way of understanding the device. In this document, you can describe how the product looks to you as the viewer and what the main features and capabilities are. The DSD describes the product using quantitative information. In other words, you use numbers to describe your product by reporting measurements and any accompanying errors concerning the medical device. The main advantage to using a DSD is that it gives numerical data on essential features of the product such as the dimensions so you can imagine what the product looks like and how it performs. A DID document will not report on how well the product performs but it gives the reader a different perspective because you can describe what the product does without giving a dry description as well as the aesthetics of the product.