Which default Reference Set is available if no additional reference sets have been defined?

The default Reference Set available when no additional reference sets have been defined is "Entire Part." This reference set includes all components and elements of a part or assembly without any limitations. It serves as the baseline reference set for users involved in design and modeling processes within the NX environment.

The "Entire Part" option ensures that all features, bodies, and components are accessible for editing and viewing purposes, facilitating comprehensive design workflows. Since it's the only set incorporated by default, users can rely on it for complete visibility of the design without needing to create or modify reference sets initially.

Other options like "Partial Part," "Selected Components," and "Subassemblies" represent more restricted views or specific groupings of components and would require the user to create these reference sets, which isn't necessary when starting from an unmodified state. Such focused sets serve specialized functions but are not the default when beginning a design in NX.