In my opinion, if you were to write a document on how to make a device, and give that document to someone else, they should be able to build an exact replica without coming back to you for questions. This is the basis of a design input or requirement. Each requirement would describe a component inside the device. It would list things such as the material it is made of, dimensions, electrical connections (if any), tools made to shape or assemble the component, and mechanical properties. The requirements should also include tolerances. For instance, if I say that a component in my device should not exceed 4 lbs, I also need to include an acceptable tolerance that would not hinder the effectiveness of my device (i.e 4 lbs +/- 0.1 lbs). In sum, this document is almost like a recipe to building your device. Without it, major safety issues can arise if the design of the device is not well defined.