It seems to me that where we are failing to communicate is that the first generation mask of a camera (photographic) negative will be a positive. On a camera (photographic) negative, the shadows are the lowest density. Therefore, when placing the camera negative in register and contact printing with unexposed lithographic film, the shadow (low density negative regions) will be the first to expose the lithographic film. This exposure of the lithographic film would be represented by density on that film.
Ultimately the aim is to have three litho film masks which are clear (no density) in the areas of the desired densities (shadow, midtone, and highlight) which exist on the camera (photographic) negative to expose the enlarging paper. All other densities apart from these must be blocked by high densities on the lithographic film masks.
The Stouffer 21 step tablet is a calibrated tablet of 21 densities separated by values of .15 or 1/2 stop. Each stop is a log value of .30 or a doubling or halving of the amount of light which is exposing the film or paper. Thus this tablet will cover 10 1/2 stops of exposure.
Peak density is the highest density which would exist on an unsharp mask. An unsharp mask is a low density unsharp positive of a camera (photographic) negative. It is a continual tone mask. It is for this reason that I use dilute (1-30) Dektol to develop the half tone ortho-litho film. The highly dilute Dektol will cause the high contrast litho film to act as a continuous tone material.
Zones are the terminology which Zone System photographers refer to as stops of reflected light defined within a scene or object. The lowest would be Zone I which has a density of .10 above film base plus fog (unexposed but developed film)--it would be defined in a print as the deepest black possible and would not represent any texture. Zone II would be the next stop (doubling) of light exposing the film. This progression continues to Zone X which is typically represented as paper base white with no texture represented. The highest Zone which we normally concern ourselves with is Zone VIII which is the highest value showing texture within a print. This Zone (VIII) is typically targeted to a camera negative density of 1.25, when printing with a diffusion light source enlarger.
The characteristic curve of a film or of a paper is a depiction of the densities, placements of densities, and degree of slope of the contrast within that materials design. It would best be described as an elongated and gradual "S" shape. The lower and upper terminus of this shape are what are commonly termed the "toe" and the "shoulder" of this curve. It is in these regions that the slope is diminished from that represented on the "straight line" section that joins these two regions. Because the degree of slope in the upper and lower terminus are reduced the contrast, and by consequence, the tonal separation is not well differentiated.
I hope that I have cleared some of the miscommunication that seems to be occurring and that I have answered your questions.