In this paper the relevance of Charles Sanders Peirce’s work in the realm of digital knowledge organization (KO) as a contribution to the challenges of the knowledge or information society is demonstrated. In particular his groundbreaking ideas about the diagrammatic aspect of relational logic are crucial and present him as a pioneer of Strukturwissenschaft (Formal Sciences). Following an operational definition of iconicity it is argued that knowledge organization systems (KOS), which formally describe the properties of objects and their relations to each other, always have an iconic dimension that is even more obvious in digital environments. To point out this aspect the development from traditional KOS to digital formal languages is elaborated in order to introduce the area of appliance. The sophisticated metadata structure of the cultural heritage database Europeana as well as simple visualizations including the relations of artists serve as examples in this paper. The leading question is: How can this research facilitate the development of KOS going beyond existing formal constraints? With the logician Sun-Joo Shin it is argued that iconic ambiguity is in fact a simultaneous plurality and does not necessarily end up in contradiction. Referring to Peirce’s Existential Graphs (EGs) Shin proposed a productive differentiation between formal and graphical logic based on the parameters of accuracy and efficiency while Iconicity is enabling the possibility of multiple readings.