When is the three-pronged IAU rule considered the definitive standard for classifying a celestial body, as opposed to a geophysical definition?
When classifying bodies within our solar system according to current international protocols.
The context dictates which definition holds authority. The International Astronomical Union (IAU) rule remains the internationally accepted protocol specifically for classifying bodies located within our solar system. If an astronomer is working within the established framework for Mercury through Neptune and the trans-Neptunian objects, the three criteria (orbit Sun, round, clear orbit) are non-negotiable. Conversely, when discussing exoplanets or seeking a definition applicable universally across the galaxy, a geophysical approach based purely on mass and size is generally preferred because the orbital constraints of the IAU rule become inapplicable.
