The purpose of the design input document is to define each user need as an objective and measurable design input. While developing the product each of these inputs must be addressed and included in the design specification document. This document should have all of the necessary details required such that another company can pick up the document and create the same product.
There should be no ambiguity in any dimensions or functions listed in the document. The design input document should be completed early on and will have little to no changes. In contrast, the design specification document will often have a number of TBD’s that will be filled in as the product is developed and tested. The two documents serve different purposes.