Empirical functions describing the dependence of the ionisation parameter on the column density of the absorbing gas. Left panel: Assumed dependencies of the ionisation parameter, log ξ, on the column density, N_H, w. Middle and right panel: Simulated tracks for a warm absorbing gas, assuming log ξ = log(100/[N_H, w/10²² cm⁻²]) (middle panel), and $log\xi =log\frac{10+[N_{\mathrm{H},\mathrm{w}}/{10}^{22}{\mathrm{cm}}^{-2}]}{[N_{\mathrm{H},\mathrm{w}}/{10}^{22}{\mathrm{cm}}^{-2}]}+1$ (right panel). The different shades correspond to different values of covering factor as reported in the labels.