as requirements analysis occurs, a set of "concerns" is uncovered. These concerns "include requirements, use case, features, data structures, quality-of-service issues, variants, intellectual property boundaries, collaborations, patterns and contracts".
Ideally, a requirements model can be organized in a way that allows you to isolate each concern so that it can be considerd independently.
In practice, however, some of these concerns span the entire system and cannot be easily conpartmentalized