We have developed a platform running on a large multi-touch and multi-users table. Its intended application domain is the collaborative preliminary design phase of engineering projects. We also developed an ontology describing the virtual objects of the platform and the projects on which users collaborate. In this paper we first present the originality of this ontology. It extends the DOLCE-CORE ontology in order to model the true nature of the virtual objects that the users can manipulate on the device and the users’ actions. We then present the different roles played by the ontology in the development of the platform. It serves as a reference of the semantics for the designers of the collaborative platform and as a model of the environment including table, participants, projects, etc. This model is mainly used by the intelligent agents of a multi-agent system whose objective is to support the participants during a working session around the table.