What does this equation mean?

The clearest illustration of why the ordering matters arrived while the paper was being finished. Roberto Iaria and collaborators reported, in data taken hours after a carbon superburst on the ultracompact binary 4U 1820-30, an absorption feature at 3.8$\pm$0.05 keV, with a width and equivalent width consistent with a real spectral line rather than noise, at a significance the authors themselves describe across their observations as ranging from 2.5 to 8 sigma [ 10 ] . Interpreted as a gravitationally redshifted iron line, the feature implies 1+z=1.72$\pm$0.05 , a compactness near 0.33, and — worked through the same exterior relation as any other neutron star — a mass of roughly 1.8 to 2.3 solar masses and a radius of roughly 8.3 to 11 kilometres. The authors describe their own interpretation as still tentative, and they make no equivalence-principle claim about it at all.