This is a small study into character spacing or tracking and how it differs from kerning.
The spacing between two characters in a proportional font is a combination of adjoining left or right side bearings, which are
values set according to the shape of each character and its relation to the shapes of all others within that font.
In the case of automatic spacing, each character is measured against
every other, both on the left side and the right side, with the goal of establishing
visually-even separation between any set of characters used to form a word.
In this example, you can see a visual representation of the separation measurements between a lowercase 'a' from
the font 'Didact Gothic', and a vertical line on the left side:
The plotted lines represent best fit curves.
In this example, you can see a lowercase 'b', measured against a line on the right:
Finally, this is an example of an uppercase 'B', with an uppercase 'V' on the right side:
The curves shown in the graphs, or similar approximations in numerical form, are used to provide average
separation distances in calculations for left and right side bearings appropriate for each glyph in a font when paired with any other.
Hopefully this provides some insight into proportional spacing and how it differs from kerning,
which is more the application of data specific to character pairs anywhere letter spacing or tracking
fails to allow for peculiarities in the way one glyph fits with another.