Currently the histogram tool only allows to visualize shadow and highlight clipping.
Enhance the tool to display the luminance zones:
Don’t forget to vote for this feature if you want it to be added! 
Someone recently pointed out that Silver Efex Pro in the Nik Collection has a luminance zone tool (see here). Can you explain a bit more what the usefulness of displaying luminance zones in PhotoLab would be? I can imagine one or two uses, but anything you can say about why we need this feature would help increase the vote count and ultimately convince DxO to implement it in a future release.
For more information you can refer to Silver Efex documentation and to this article :
There is value in bring the tool to any photograph, color or grayscale, directly in Photolab, without the need to export to Nik tools.
For fun and analyzing it has some value for me. But when you read that article you will see that the zone info is nowhere used. It tells/shows on what part of the histogram the different tools are working. And even that is not equal with other editors like that one in PL.
Knowing that those parts that are to dark for me are laying in zone 0 or 2 or 3 doesn’t tell me what tool to use.
The zone system is part of the analogue time.
George
Thanks George. For me, I think zones could be user-defined in 2, 3-, … n-zones.
This way of creating masks is offered in Picture Window Pro for letting a user define two or three zones. I have found this feature useful in PWP. This feature would be another way to define masks in the local correction tool.
For PWP documentation, follow these links:
https://www.dl-c.com/Documents/WebHelp/index.htm?context=1480
https://www.dl-c.com/Documents/WebHelp/index.htm?context=1480
Please excuse the ramblings of an old film photographer…
The Zone System was originally intended as a way of correctly exposing a sheet of film in such a way that high dynamic range and low dynamic range subjects would print well within the limited range of “darkroom” printing paper.
It involves “placing” deepest shadow at such a point on the exposure range that it would contain discernable detail. Then calculating the overall range of exposure to determine by how many stops you needed to “push” or “pull” the exposure and processing to produce a negative that is “easier” to print.
Digital can use a similar technique but, being a “positive” image, the driving factor changes from being deepest shadow with detail to brightest highlight with detail.
You need to test your camera to find out just how far you can over-expose a highlight area without losing detail. With my Nikon D810, that works out at between 1â…” and 2 stops. Now I know that the sensor is capable of recording up to 14 stops of range at 100 ISO, so I can now manually spot measure the brightest part of the image and leave the rest to fall where it will within the range of the sensor.
Some people refer to this technique as ETTR (expose to the right) and should produce a histogram that is never blown on the right hand side but that may well look under-exposed on the left, especially if it is a RAW file.
To display such a scale on the histogram would involve having to calculate where the true “zone 0” is, as it could easily be well to the left of the “visible” histogram. See how FastRawViewer allows you to reveal detail in the deepest shadows.
I don’t like the idea of a simple 1…10 scale, as often we can be dealing with more than that.
What I would rather like to see is black and white point sliders below the tone curve (like in PS) where you can determine where the curve range should actually start and finish.
In my opinion, actual zone numbers are not really all that relevant to digital, mainly because we are no longer dealing with a fixed range output to paper.
That’s not a zone system. More a range definition.
Could be something to think about.
George
I really like the curves and histograms “control points” in Picture Window Pro, with before and after arrowheads shown on two connected histograms with possible expansion of the histogram vertical scales.
Just discovered this thread and had a 
Off and on I start SilverEfex for the sake of the zone system inidcator (I’m printing …)
Wolfgang