Ok, it is more clear now to me.
So the curve is logarithmic (EV in horizontal axis) and is adapting the input to an output that is more similar to how human perceive lightness.
That is why it seems linear in the midtones (inside latitude) but really it is not.
It does not represent output versus input (as in most curve manipulating programs), but versus log2 input.
May be it is easy to make it user selectable (I have discovered you can change it in base curve) and so you can have the typical output versus input curve.
At the same time it is compressing the extremes (outside latitude) to fit it in an output DR (which one, our display profile, export profile, print profile? there are many selected at the same time).
I think it is a bit confusing and what causes most of our misunderstandings.
Which is the profile that filmic is using and the DR it is considering when you work in the screen tab?
OK, I understand that you are not mapping to the output device, a linear mapping is done in the final step in the output profile (or conversion to display) transformation.
We can think of filmic as having a virtual output device (or just transforming to the 0.0 to 1.0 output).
But still, it does the compression outside the latitude to compress lights and shadows.
But the quantity of compression you need depends on the real DR of the output device related to input.
I there is enough DR in output to your DR in image, you do not need to compress anything, but if you change the device, to a less capable one, there you need to decide best way to compress it and what to preserve.
I have the impression that filmic is biting too much things at the same time: a transformation that make things more similar to human perception and a compression that fits the DR to an output DR.
It would be easier to understand (and select what you want to do and what you don’t) to do that in two steps.
It does a great job with images that have a wide DR and you want to display them in a standard sRGB monitor with 8 EV, for example.
But what happens if you have a display with 10 EVs?
And if you want yo print it in a printer with lower DR and quite different colors?
I suppose you should change parameters in filmic to adapt the output to that printer.
And that is one of the problems I anticipate with making two works at the same time.
You may have applied some processing after filmic, because that processing is better done in a space more close to human perception (vignetting in perceptual uniform way, for example or changes in colors or saturation).
If it were separate steps, with a last step for mapping just before output profile you have to change nothing if you change the output media, but tweak that last compression step.
The other kind of images where I don’t see clearly how to work in a linear + filmic workflow is with images that have say 4 o 5 EV in origin, filmic still is useful to adjust contrast and have an output similar to human perception.
But it keeps trying to compress the range in the extremes when there is no need for that, as a pure logarithmic transform would fit perfectly our input range in the output range.
In that situation many times you don’t want to compress the DR, but to expand it.
But (as long as I know) you cannot get rid completely of the curved zones in the extremes using filmic.
If I have enough dynamic range in my output device to show the DR of the imagen, I don’t need any compression of lights or shadows, and if I apply filmic there is always some.
You can minimize it, but it is still there.
And if you change the output device, for one with larger or narrower DR you have to change filmic curve in order to fully exploit the dynamic range of the output device wihout unnecessry loosing or desaturating colors
If the process were split in to phases, you just need to change the final step of comprising DR, and the contrast and gray point may be adjusted in a previous curve to get the logarithmic / perceptual related output.