Essentially, all models are wrong, but some are useful. —George E. P. Box SAFe Requirements Model Note: This article is part of Extended SAFe Guidance and represents official SAFe content that cannot be accessed directly from the Big Picture. To support bringing the benefits of Lean and Agile development to larger enterprises—or smaller businesses building more complex systems—SAFe provides a scalable requirements model that demonstrates a way to express system behaviors: Epics, Capabilities, Features, Stories, Nonfunctional Requirements (NFRs), and more. As shown in Figure 1, each of these work items is expressed differently.