Systems-of-systems are characterized by the independence of their constituent elements. Such an element is usually socio-technical, comprising technology, humans, or organizations. To capture its independence, it needs to be viewed as an intelligent agent that relies on an internal model of the world for its decision-making. Hence, a system-of-systems model will include multiple agents that inside themselves contain different models of the same system-of-systems. Describing these overlapping subjective models and their usage by the agents is essential to properly understand the resulting behavior of the overall system-of-systems. Current modeling practices are not well suited for dealing with this, and the paper therefore outlines an ontology that makes the agents and their internal models more explicit. The paper also discusses the implications such models have on systems engineering practices and how they address known system-of-systems engineering pain points.