Ensuring the FAIRness (Findable, Accessible, Interoperable, Reusable) of data and metadata is an important goal in both research and industry. Knowledge graphs and ontologies have been central in achieving this goal, with interoperability receiving much attention. The European Open Science Cloud initiative distinguishes in its Interoperability Framework (EOSC IF) four layers of interoperability: technical, semantic, organizational, and legal. This paper argues that the emphasis on machine-actionability has overshadowed the essential need for human-actionability of data and metadata, and provides three examples that describe the lack of human-actionability within knowledge graphs. In response, the paper propagates the incorporation of cognitive interoperability as another vital layer within the EOSC IF and discusses the relation between human explorability of data and metadata and their cognitive interoperability. It suggests adding the Principle of human Explorability, extending FAIR to the FAIREr Guiding Principles. The subsequent sections present the concept of semantic units, elucidating their important role in enhancing the human explorability and cognitive interoperability of knowledge graphs. Each semantic unit can be displayed in a user interface either as a mind-map-like graph or as natural language text. Semantic units organize knowledge graphs into levels of representational granularity, distinct granularity trees, and diverse frames of reference. Semantic units can be represented as FAIR Digital Objects (FDOs) that instantiate corresponding FDO classes. Various categories of FDOs are distinguished. The development of innovative user interfaces enabled by FDOs that are based on semantic units would empower users to access, navigate, and explore information in knowledge graphs with enhanced efficiency.

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