¹²CO and ¹³CO (top) with HCN and HNC (bottom) excitation ladders of NGC 253 with flux of each transition plotted as black asterisks with red error bars. In dotted blue, green and yellow lines we plot the PDR/mCDR/mCDR (left), PDR/mPDR/mCDR (center), and PDR/mPDR/mPDR(right) ISM phases with their filling factors. The composite model is plotted with a solid red line. The model H₂ density [log cm^-3], G₀ [log Habing flux], and percentage mechanical heating (α) are shown in the legend along with the relative contribution of each phase in terms of emission and column density.