The other thing that is involved with the characteristic curve of a film is the left vertical line of the characteristic curve graph is the net density, the lower horizontal line is the measurement in stops of exposure (zones). Therefore in comparing characteristic curves of films we can determine the length of the toe, where the shoulder occurs, and how pronounced it is.
The reason for looking at the length of the toe is that since this is the shadow area of the film. We can look at this in terms of stops of light (zones) that are found on the toe. This is an area that does not separate tonalities well since the angle of the slope is much shallower then the straight line section that then follows.
When we view the straight line, we can determine the severety of the angle (the amount of density added per stop of light exposure). This will depict the separation of the midtone values.
When we view the shoulder we see at what point it begins to be introduced in relation to stops of exposure and how much it flattens. If it flattens markedly then any highlights that are recorded there will not be separated well in their tonality.
If one were to compare a high contrast film such as tech pan we would see that this film has a fairly straight line from the toe to the shoulder. This film can tolerate a great deal of contrast expansion. (depending on the developer used and time of development). Conversely Tri X has a fairly long toe (reaching almost to zone III) and then has a fairly long straight line (zones III-XI). This film would not separate shadows well if it is exposed so that our lowest value is placed at a Zone I value.
However, we can expose Tri X so that our low values are on Zone III. Because the exposure (in this example) has placed our low values on the straight line section of the film curve the shadows are better separated.
Just as there are characteristic curves for films there are also characteristic curves for papers. In the case of paper, the toe area would be exactly reversed from film in that the highlights are represented on the toe and the highlights are represented on the shoulder.
I think that a great deal can be learned about the characteristics of materials by viewing the manufacturers graphic depiction of those characteristics. Ultimately, of course, the defining parameter is what is shown in the print. Understanding our materials helps that process along.