Broad C IV λ1549 EWs versus ionization parameter log U from sub-Eddington CLOUDY models (parameters in Table 1). Line styles indicate different broad-line region CFs; colors distinguish SEDs. The downward-pointing red arrows mark the 3σ upper limit from the z > 5 grating stack. For consistency with our measurements, only the stronger CIV λ1549 component from CLOUDY is shown. The vertical axis uses a symlog scale (linear below 10 Å, logarithmic above).