Incomplete definition of Concept?

On https://pubs.opengroup.org/architecture/archimate32-doc/ch-Language-Structure.html section 3.2 we can read this definition of 'Concept': A model is a collection of concepts – a concept is either an element or a relationship. My interpretation is that there are two kinds of 'Concept', namely 'Element' and 'Relationship'. And that these two kinds are mutually exclusive. However in Figure 1. Top-Level Hierarchy of ArchiMate Concepts underneath I see a third kind of 'Concept', namely 'Relationship Connector'. In daily practice with ArchiMate I don't really mind the (assumed) incompleteness of the definition. But today I had the idea to put the official conceptual model and vocabulary with definitions in our EA-repository and connect it with the many custom repository data models that exist for more structured analysis and design. Can you include 'Relationship Connector' in the textual definition of 'Concept'?