  Using models for requirements engineering (RE) is uncommon in systems
engineering, despite the widespread use of model-based engineering in general.
One reason for this lack of use is that formal models do not match well the
trend to move towards agile developing methods. While there exists work that
investigates challenges in the adoption of requirements modeling and agile
methods in systems engineering, there is a lack of work studying successful
approaches of using requirements modelling in agile systems engineering. To
address this gap, we conducted a case study investigating the application of
requirements models at Ericsson AB, a Swedish telecommunications company. We
studied a department using requirements models to bridge agile development and
plan-driven development aspects. We find that models are used to understand how
requirements relate to each other, and to keep track with the product's
evolution. To cope with the effort to maintain models over time, study
participants suggest to rely on text-based notations that bring the models
closer to developers and allow integration into existing software development
workflows. This results in tool trade-offs, e.g., losing the possibility to
control diagram layout.
