Comparison of key parameters to differentiate between the ice-only and ice+rock scenarios for Neptune and Uranus. Left: Envelope metallicity. Middle: Pressure at the transition between the envelope and mantle. Right: J₄ gravitational harmonic, including the corresponding measured value (dashed line) and its 1σ error range (dotted lines). Bottom subpanels: Box plots summarizing the distributions, where the central vertical line indicates the median, the box represents the IQR, and the whiskers extend to values within 1.5 times the IQR.