|The Business Process Model and Notation (BPMN) is a widely used standard for business process modeling and aims at business analysts and technical developers. In contrast to several other business process modeling languages, BPMN provides explicit concepts for decomposition to cope with the complexity of business processes and reusability to enhance the consistency of process models. In particular, BPMN 2.0 specifies the elements sub-process and call activity for addressing decomposition and reusability, however, the specification shows some contradictions and limitations. In this paper, we study the BPMN constructs for decomposition and reusability in detail and formally define the syntax within an ontology. The definition points out contradictions in the BPMN specification concerning the class hierarchy and attributes of sub-processes. Furthermore, open issues have been identified, i.e., the instantiation of sub-processes and call activities, the use of the constructs in case of unstructured diagrams or to combine tasks being distributed across different swimlanes, and the differentiation of synchronous and asynchronous decomposition. As a result, we will provide recommendations for the definition of sub-processes and call activities and practical suggestions to handle each of the identified problems.