Internet of Things (IoT) is now pervasive in most business and Public Administration processes. Along with the dizzying development of technological solutions, in recent years new methodological approaches are emerging with the objective of abstracting IoT functionalities, in order to manage them as resources in project management methodologies. A critical aspect is the representation of the knowledge grasped from the data acquired by IoT devices, since different types of users interact with such data with different goals. In order to transform them in knowledge, data have to be organized in a proper way and meaningfully provided in an IoT application specific for that type of user. In this paper, we propose the Knowledge Stratification Model that technical experts can take into account when developing an IoT application. The model, which organizes the knowledge elements in three layers, aims to identify the data produced by IoT devices and integrate them into business processes, thus making them meaningful for the user. A semantic approach, based on three subsets of ontologies specific for each model layer, is proposed to represent domain knowledge and to solve the technological and user interaction semantic issues characterizing complex and heterogeneous contexts as Smart City.