The Design Input Document (DID) and Design Specification Document (DSD) are very similar, but I think they belong to distinctly different steps in the product development life cycle. The DID is meant to be just the big picture type of document where the marketing department goes out and does research to find a market gap or surveys medical professionals and determines the basic customer needs for a new product. These are general requirements and are sort of just like a brainstorming session where you get all of your ideas on the table. The DSD is where the engineering department comes in and determines what is actually possible given current technology and resources, and designs the detailed specification for the product. The information in the documents is the same, but it is prepared by two totally different departments of vastly different expertise. Both are useful in visualizing the product, but one is a means to create the other.