With their growing volumes and uses, it is increasingly important to understand the interpretative and representational aspects of three-dimensional (3D) geosciences models. Such understanding will not only clarify key premises, inferences, and conclusions, but also enable more informed applications. Yet the epistemic foundations are often opaque. Critical information about assumptions, reasoning steps, and uncertainties typically remains tacit in the mind of the geomodeller. This lack of transparency hampers explainability, reproducibility, and broader utility. Current practices therefore 15 limit trust, knowledge transfer, and automation in geomodelling workflows. To address these limitations, we develop the POKIMON ontology, designed to make explicit the expert knowledge, interpretative choices, and conceptual structures underlying 3D geosciences models. POKIMON provides a formalized framework to represent how geological and geomdelling concepts are applied during model construction. Motivating use-cases, the ontological structure, and its application to the use[1]cases are presented to demonstrate utility and to advance automated knowledge-driven 3D geomodelling.