Chroma QP mapping tables are modified such that the relationship between luma PSNR an chroma PSNR is on average closer to the one observed with HM 16.20. The net effect is that bits are shifted from chroma to luma and BD-rate differences between VTM and HM are most consistent across components.
BD-rate between VTM 8.0 and HM 16.20: AI: -24.09% -31.32% -32.82% RA: -34.84% -46.33% -46.91% LB: -30.02% -42.62% -40.94% LP: -33.12% -44.28% -42.80%
BD-rate between VTM 8.0 plus this change and HM 16.20: AI: -24.99% -25.35% -26.86% RA: -35.76% -39.04% -40.03% LB: -30.56% -32.89% -30.77% LP: -33.62% -34.86% -32.56%