  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.
