System Engineering (SE) is a multidisciplinary activity in witch a system engineer team aims at building a global technical solution in response to a problem stated as a set of requirements. SE may be seen as process of optimizing some system quality factors under constraints of required functions, cost and delay. Complex products and systems, involve a large number of components which are arranged under constraints/relationships in engineering space. This may lead to uncontrolled and unpredictable backtracks in the engineering processes. Effectively managing such process is a major factor witch can increase product development capability and quality and reduce the development cycle time and cost. In this paper, we propose a formal approach for optimizing component allocation and parameter configuration in SE process. We introduce the concept of Alternative Quantity (AQ) to enable manipulation and reasoning on partial engineering choices. The main contribution is to offer efficient evaluation of global systems proprieties although particular technical choices have not been done yet.