Glyph substitution replaces selected glyph indices by other glyph indices while laying out text. In OpenType, the GSUB table specifies these replacements after the default character-to-glyph index mapping. A substitution can replace one glyph by another, several by a typographic ligature, or one by several; its application can depend on surrounding glyphs.[ref]
Glyph Substitution Table by Microsoft. Overview and Lookup Types1–6: input/output glyph indices, ligatures and contextual substitution
For a hypothetical computer font, suppose
f and i initially select indices17 and23. A matching ligature rule could replace the pair by index91. These numbers belong to that particular computer font; they do not change the stored Unicode sequence into code point91. The example also shows why substitution differs from kerning: replacing the pair selects a new shape, whereas adjusting the pair's positions can retain both original shapes.Lookup order matters when one substitution creates or removes the input pattern for a later substitution. Text shaping must therefore interpret the applicable OpenType rules, rather than independently choosing a default glyph for each text character.
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