A design specification is a detailed document providing information about the characteristics of a project to set criteria the developers will need to meet. Its use is called for where a structure or product has to be specially made to meet a need. For example, a design specification must include all necessary drawings, dimensions, environmental factors, ergonomic factors, aesthetic factors, cost, maintenance that will be needed, quality, safety, documentation and description. It also tells specific examples of how the design of the project should be executed, helping others work properly.(A guide line for what the person should do).
References
Design specification Wikipedia(Text) CC BY-SA