There is no relationship between the Bitrate and Quality sliders.
The Quality slider controls a number of parameters related to motion searching, all of which can have a significant effect on quality. Bitrate can also have a significant effect on quality, but it is an important enough setting that it should have its own control. This is why the Quality slider controls everything—except bitrate—which can affect quality.
For example, many people want to keep a certain bitrate to fit their content onto a disc, but then they want the highest available quality at that bitrate. If the Quality slider also controlled bitrate, those users would be out of luck. If they set the slider higher, they would not meet their bitrate budget. If they set it lower, they would have the perception of only being able to use a mediocre quality setting.
With the current implementation, a user can set the target bitrate (4k, for example) and then crank the quality up without affecting the bitrate setting. This means that the encoder will do the most diligent job with motion searching and the other things (except bitrate) that affect quality. And it is very possible to get pristine quality at lower bitrates using this approach—with the Quality slider independent of the Bitrate slider.
The Quality slider works on a threshold basis. It tells the encoder how far to look for certain kinds of motion before making a decision. In most cases, the 2–3 range (on a scale of 5) provides more than an adequate threshold. This means that lower quality setting doesn't actually mean lower quality in all cases because of this threshold approach. Challenging material needs more diligent motion searching; that's where the Quality slider can really make a difference.