The anchor, or baseline, is the encoder that every other configuration is measured against when reporting BD-rate. Because BD-rate is always a difference between two encoders, a comparison needs a fixed reference, and the anchor is it. A common choice is x264 at its medium preset, the universal baseline that runs everywhere, so a BD-rate of minus 40% means a given configuration reaches the same quality as x264 medium at 40% less bitrate, while plus 5% means it needs 5% more. The sign convention is the constant source of confusion: negative is a saving against the anchor. The catch is that the whole comparison only holds if it is apples-to-apples - the same content, the metric and model, the resolution handling, and the speed effort held comparable across every configuration and the anchor. Change the anchor and every reported number changes with it, which is why the anchor and its exact version must be named in any result.
