What does this equation mean?

What a moment-of-inertia factor rules out is more reliable than what it establishes. It is a single number, so it cannot distinguish a small dense core under a light mantle from a larger, less dense core under a heavier one; infinitely many radial density profiles share the same $I^{*}$ . But it will refuse an entire class of models outright. For Mercury, MESSENGER radio tracking combined with spin measurements gave C/M$R^{2}$ = 0.353 $\pm$ 0.017 together with a ratio of mantle-plus-crust moment to total moment of 0.452 $\pm$ 0.035 , a combination consistent with a solid silicate outer shell over an iron-sulfide layer, a liquid iron-rich outer core and possibly a solid inner core [ 7 ] . Seven years later, a longer tracking arc revised the normalised polar moment to 0.333 $\pm$ 0.005 [ 9 ] . The revision is worth dwelling on: the earlier value was not wrong within its stated uncertainty, but the interior models it supported shifted materially when the uncertainty shrank by a factor of three.