ISO/IEC 33004:2015 sets out the requirements for process reference models, process assessment models, and maturity models. The requirements defined in this International Standard form a structure which specifies a) the relationship between the classes of process model associated with the performance of process assessment, b) the relationship between process reference models and prescriptive/normative models of process performance, as constituted by, for example, the activities and tasks defined in ISO/IEC 12207 and ISO/IEC 15288, c) the integration of process reference models and process measurement frameworks to establish process assessment models, d) the use of common sets of assessment indicators of process performance and process quality in process assessment models, and e) the relationship between maturity models and process assessment models and the extent to which a maturity model can be constructed using elements from different process assessment models.