Reuse and requirements engineering are very important for efficient and successful systems development. However there are many open issues for performing them well in practice, especially raise of requirements. We present a new approach to reusing requirements based on application families i.e. several systems in a given problem domain. In contrast to precious approaches, an application family model in our approach consists of a pool of requirements (linked in a lattice structure), a domain model and a set of discriminants (a special kind of requirement that differentiate one system from another). We focus on how to make an efficient and clean selection of the requirements for a new system from an application family model. We have developed a method for solving this problem and a prototypical tool for supporting it. The method and the prototype were successfully developed under a study contract for ESA (European Space Agency). We present a case study, where we generated commanding requirements for an individual mission from an application family model of commanding requirements for spacecraft control operating systems. As a consequence, we propose this approach for reusing requirements based an application families.