Clause 6.1 opens the modelling part of the document. It sets out the general considerations that govern the diagrams in clauses 6.2 to 6.4 and the identifier construction in clause 7. The public contents show that the modelling material moves from a conceptual overview, to a high-level diagram, to a detailed description with relationships between classes, attributes of classes, generalised classes and patterns, and translation and language. Clause 6.1 is where the document explains why it models the domain the way it does, rather than listing fields. The detail is not in the public preview, so the specific modelling rules are not stated in the text we hold. What the structure shows is that the document expects an implementer to work from a class model, not from a flat table, and to keep the model separate from any one message format.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.