I believe they are both necessary because they help you take action. These documents basically tell you what you will be building. With these documents technically anyone should be able to build your product. They both work off each other as was stated the DID is just a broad description so they are not any numbers or specifications. The DSD has all the numbers and the specifications. But you cannot have a DSD without a DID. They are both really helpful because as I stated the words in these documents will lead you to the actions you need to make the product. For example “red knob, 3-cm in diameter, dyed with Red #5 from Vendor X or Vendor Y” this statement from a DSD tells you the exact color and size of the knob and it also tells you where it will be going. With these words you can take action and being to build your product.