Standard deviation σ₈₅₇ of the residuals with respect to the Planck-Hi correlation at 857 GHz versus the mean N_HI, both computed within circular sky patches with 5° diameter and over unmasked pixels. The red hatched strip marks the contribution of CIB anisotropies to the residuals at 16′ resolution, computed from the CIB model in Planck Collaboration XXX (2014). The width of the strip represents the expected scatter (± 1σ) of this contribution. Both the scattered distribution of data points above CIB anisotropies strip and the increase in the mean σ₈₅₇ with N_HI arise from residuals with a Galactic origin (Appendix D).