what is design specification??

Design specifications, like blueprints, provide detailed guidance on what the project outputs will look like and how they will be expected to perform. The key difference is that design specifications provide that guidance through the written word, coupled with graphics and drawings, whereas blueprints are strictly graphic in nature.

The purpose is to provide clear direction to the project organization on what the final outputs of the project must look like and the tolerances and standards those outputs must meet.
Application
Design specifications are used as soon as they are available to determine some components of the work to be performed and to prepare for the purchasing and allocation of materials to the project. Because design specifications incorporate information about certain performance standards, the specifications can even be helpful in determining which resources are best suited to assist in or perform some of the work, because some work requires more highly specialized workers than others. The design specifications are used to flesh out the customers’ functional requirements and technical requirements, expressing specifically how those needs will be addressed by the look and feel of the final deliverables. The components of the design specification should be traceable back to the functional and technical requirements, and there should be evidence that each component of the design specification will be addressed by some component of the work breakdown structure.

In deliverables-oriented organizations (in contrast to service organizations), the design specifications often become the document used to define project intent, expectations, and commitments. They are often referenced in project litigation as the rationale for certain approaches.

Considerations
Design specifications have their roots in industry where physical deliverables are produced. As such, the orientation with design specifications is to provide physical descriptions of objects, components, and all aspects of the deliverable. Design specifications can still be employed on service projects, however. By defining how processes should perform and graphically representing any specific needs of the customer organization, the same processes that have served product industries for decades can be applied in those organizations providing services, rather than products.