White Point Setting
The curves dialog box allows you to view value information in RGB (0-255) or ink percentage (0% to 100%) values.
Value Readout in Ink Percentage
Light (0-255 RGB) vs. Ink Percentage Readout
I recommend viewing the curves dialog box value information readout in ink percentages simply to make the relationships easier to understand. It is easier to comprehend percentage changes on a scale from 0 to 100 than on a scale from 0 to 255. Ink percentage is a 0 to 100 scale.Light readout is a 0 to 255 scale.
White Point, Black Point, Mid Tone, 1/4 Tone, and 3/4 Tone
Five Tones of the Tone System
When we use the Tone system for color correcting, we are concerned with five specific tones: White Point, Black Point, Mid Tone, 1/4 Tone, and 3/4 Tone. Adjustment of these five tones encompasses the first five steps of the color correction routine.
(Though I enjoy the nod to Ansel Adams' "Zone System" in the name "Tone System", these tonal points are actually the source of this workflow's name.)