Replace glyphs designed to be set on full-em widths, fitting them onto half-em widths using the 'halt' font feature.
Don't apply any special glyph-width features.
Replace glyphs set on other widths with glyphs set on full (usually em) widths using the 'fwid' font feature.
Replace glyphs other than half an em, with glyphs on half-em (en) widths using the 'hwid' font feature.
Respace glyphs designed to be set on full-em widths, fitting them onto individual (more or less proportional) horizontal widths ('palt').
Replace glyphs set on uniform widths (typically full or half-em) with proportionally spaced glyphs using the 'pwid' font feature.
Replace glyphs on other widths with glyphs set on widths of one quarter of an em using the 'qwid' font feature.
Replace glyphs on other widths with glyphs set on widths of one third of an em using the 'twid' font feature.
Specifies the preferred glyph widths (generally used in CJKV fonts).