UML Class diagrams are widely used for purposes such as meta-modeling and database and ontology engineering. This paper opens with a concise presentation of the class diagram model, which is followed by an applicative illustration concerning the “Engineering Unit Operation” knowledge. A taxonomy diagram of this sphere of knowledge, showing its concepts hierarchy, is consequently driven. I.e. a visual layout that correlates in a logical theory the physical system description to the main classes of its equations. The latters are scattered in the field of the applied Fluid Dynamics, Thermodynamics, Industrial Chemistry, Food Applied Science and Biotechnology.