The purpose of X-SCTL/MUS methodology is to support the collaborative development of software requirements specifications. It is a viewpoint-based approach which separates the descriptions provided by different stakeholders, and concentrates on identifying and resolving conflicts between them. X-SCTL/MUS allows the incremental development of each partial model, facilitating the task division and decreasing the development complexity.

In order to fulfill all these exigencies, one of its functionalities consists in providing a way to compose requirements from different agents in a merged specification which represents explicitly overspecification and underspecification phenomena and over which, for instance, it is possible to formally verify properties. The main issue in this paper is precisely the description of the composition operator used to obtain the merged model representing the stakeholders? requirements. We will show the criteria employed to select the composition operator that fits the desired characteristics best. Finally, the contribution will be illustrated with a little but explanatory example.