Abstract
This paper illustrates how semantics is modeled and used in the flexible and idea inspiring ontology-based Do-It-Yourself architecture (Onto-DIY). The semantics part contains three divisions - 1) semantics from the domain ontologies, which are used as the semantically rich knowledge for describing Internet of Things (IoT) components, 2) semantics stored in semantic decision tables, which contain decision rules on top of domain ontologies, and, 3) semantics from user-centric services, which define the software composition for the deployment.