“UNITS”: A FUNDAMENTAL MEASURE
OF SPACE AND WIDTH

In the 80s and 90s, at least, pre-desktop typesetting systems that ran on digital computers used units of 18ths, 36ths, 54ths, or 108ths of an em. 54ths was the most common. It seemed, at the time, quite adequate for fine work.

Desktop typesetting software broke away from the 18-based systems. (I’d guess that increased processing power made this possible.)
    Quark used units of one two-hundredth of an em. I found this to be a very handy unit value, not too coarse and not too fine, so that most kerning and tracking adjustments in running text required only one to three units—and one to three keystrokes.
    Quark’s units could be further divided into smaller decimal fractions, but I rarely found occasion to do so: only for fine kerning of very large display type, or for tracking very small legal type on long measures, to lose a line without tightening the already hard-to-read type any more than was absolutely necessary.
    Fontographer, long a standard desktop application for font creation and modification, used units of one thousandth of an em. So did (and does) Illustrator. InDesign followed this.
    Such a fine unit is probably useful for font creation and font editing, but it is needlessly fine for even the finest typesetting. With InDesign, I set the Units & Increments preference so that the Kerning/Tracking unit (the amount added or subtracted by keying Option-Left/Right Arrow) is five thousandths of an em—which is equal to one two-hundredth of an em. InDesign’s default keystroke value is 20 thousandths of an em, which is much too coarse—so much so that it probably deters people from working with units. (When reading or inputting numerical values for units in InDesign, you are still working with thousandths.)

For more on fine work with kerning and tracking in InDesign, see the article Controlling Text Aesthetics with Kerning and Tracking.

SPECIAL SPACES IN UNICODE AND INDESIGN

Unicode has a thin space (U+2009) that is one fifth or one sixth of an em, and a hair space (U+200A) that is narrower than a thin space. These can vary with the application used to set the type, and perhaps also with the font, at the option of the designer. (Unicode also has a 3-, 4-, and 6-to-em space, en and em quads, as well as a punctuation space, and a figure space distinct from the en quad.) The hair space, thin space, and some of the others are not meant for typographic use, but only to facilitate some conversions

InDesign has a hair space and a thin space, in addition to sixth-, quarter-, and third-spaces, and also punctuation, em, en, and figure spaces. (Quark, as I recall, had something similar.) The thin space and hair space are said to be breaking in Unicode, but non-breaking in InDesign.
    The punctuation space could be useful in some types of work, especially tabular work, since the punctuation space would take up the space of a period or comma between divisions of a large number. The punctuation space is said to be breaking in Unicode, but non-breaking in InDesign.
    I don’t know if, in InDesign, the hair, thin, punctuation, and figure spaces yield fixed widths regardless of font. (See the paragraph on Unicode, above.)
    A quick test, measuring paper output, with OpenType and non-OpenType versions of 12-point Trump Mediaeval gave the following approximate values, the same for both OpenType and non-OpenType fonts: hair space: about one sixtieth of an em; thin space: about one thirteenth of an em; punctuation space: about one-quarter of an em; figure space; about one twenty-fourth of an em larger than the en space.
    I have seen it written that InDesign’s hair and thin spaces are one twenty-fourth and one eighth of an em, respectively.

I have used InDesign’s thin space occasionally for setting French, as the space between some punctuation marks and the word they precede or follow.
    Otherwise, I have never had any real use for the hair, thin, punctuation, and figure spaces–even though I’ve done a great deal of financial work. They are probably useful only in shops where certain types of specialized work are done regularly, by people who are familiar with any tradeoffs their use may involve. I’ve always used en spaces for figure spaces.
    The thin and hair spaces might be useful for tweaking the spacing between certain character pairs—you could do a search-and-replace to apply the same adjustment throughout a long text. However, the presence of the spaces might cause problems if the same string was later included in another search-and-replace string, or if the text was moved to another application. I’ve always used hand kerning (Option-left/right arrows) for such adjustments—or, if there was a lot of work where the same adjustment was required, I’ve changed the kerning metrics in the font.