FontTracking is an option for Cell, StyleBox and StyleForm which specifies how condensed or expanded you want the font in which text is rendered to be.
Typical settings are "Condensed" and "Expanded". The default is "Plain".
For some fonts and on some computer systems, additional settings are supported, such as "Narrow", "Compressed", "SemiCondensed", "Extended" and "Wide".
See notes for FontFamily.
See the Mathematica book: Section 2.10.10.
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