Engineers distinguish between nominal and actual qualities, a dichotomy that is fundamental to guarantee that physical objects satisfy design requirements. Computational ontologies are broadly exploited across engineering domains, even though they do not attempt at making explicit the intended semantic of the two notions. The purpose of the paper is to present a foundational analysis of nominal and actual qualities to support their robust specification for knowledge representation. Instead of presenting an ontology, we discuss various modeling alternatives on which users can rely to develop their ontologies.