What do white and black on the pr palette mean?

"Highlight" is mainly used to control the brightness of 192 to 255, but it also has some influence on 128 (medium gray), and the influence will be greatly attenuated (almost unaffected) from 128 to 0. So you can understand the main control range of highlight as 128-255, and the center point is 128. But infinitely close to 255, it can be understood as gentleness when brightening, and the details of highlights can be kept as much as possible. On the contrary, darkening will increase the contrast of highlights, but it is also prone to color scale faults.

"White" is mainly used to control the brightness from 128 to 255, and the influence gradually decreases from 128 to 0. The control characteristic of white is to control the brightness in a large range, but not the overexposure of highlights, so if it is too bright, it will produce dead white, and conversely, when it becomes dark, the intensity will become flat, which can increase the details of highlights and enrich the brightness levels, and it is not easy to break the color scale.

"Shadow" and "black" are symmetrical, with highlights corresponding to shadows and white corresponding to black.

"Shadow" is used to control the brightness of 0-64, and the influence range reaches 128. The control characteristics are that the intensity is gentle when it is dark, which can make the transition natural, and the intensity is strong when it is bright, which can increase the contrast of dark parts but easily break the color scale.

"Black" mainly controls the brightness of the color scale from 0- 128, and the influence gradually decreases from 128 to 255. Its characteristic is that the brightness is controlled in a large range, and it is easy to produce dead black when it is dark, and it is softer when it is bright, which naturally increases the dark level.

How to use it depends on the actual situation. The key is to understand the scope of these two pairs of controls, as well as the characteristics of highlighting and restraining darkness, so as to foster strengths and avoid weaknesses.

I found two test curves online. You can have a look.