Hierarchy has widely been recognized as a viable approach to deal with the complexity of conceptual models. For instance, in declarative business process models, hierarchy is realized by sub-processes. While technical implementations of declarative sub-processes exist, their application, semantics, and the resulting impact on understandability are less understood yet-this research gap is addressed in this work. In particular, we discuss the semantics and the application of hierarchy and show how sub-processes enhance the expressiveness of declarative modeling languages. Then, we turn to the impact on the understandability of hierarchy on a declarative process model. To systematically assess this impact, we present a cognitive-psychology based framework that allows to assess the possible impact of hierarchy on the understandability of the process model.