Although the characteristics and cost of any technical system are highly influenced by the early design phases, little focus has been on thestructure and use of the specification in manufacturing system design, in this paper exemplified by assembly systems for large, high-volume products. This paper proposes a framework for elaborating such specifications. The framework is based on a division in qualifying and winning criteria, and is structured into four classes: functional requirements; internal design constraints; external design constraints; and winning criteria. Defining the specification by these terms is justified and explained by comparing standard mathematical formulation for this kind of problem to theframework. Future research efforts are pointed out where the framework is used throughout the development project for goal-focus, as thespecification is used and refined in an iterative manner.