Software release planning is the process of deciding what to include in future release(s) of a product. Basically the problem can be seen as a company-wide optimization problem involving many stakeholders where the goal is to maximize utilization of the often limited resources of a company and turn them into business benefit. Saliu and Ruhe have proposed a set of key aspects for release planning methods, of which only a subset have been validated in industry. In this paper we use the Saliu and Ruhe key aspects as a starting point for identifying key aspects of release planning. To do this we have performed a multiple case study involving 7 international industrial companies, all producers of software intensive products. Our contribution is 1) a more strict meaning of a release planning key aspect, 2) validation of some of the aspects proposed by Saliu and Ruhe, and 3) an extension of the key aspects. We also capture state-of-the-practice for release planning in industry.