Replace letters with lookalikes or close lookalikes in other scripts, for example Latin A to Cyrillic А. How close the lookalikes are depends on the font they're shown in (especially since some fonts don't have Greek/Cyrillic/Armenian/Cherokee letters and so the letter will be shown in a different font and stand out), but for fonts which have them, in general, the "exact match" setting will use visually identical symbols, while the "close match" set will be obviously different (using e.g. ε=e, м=m, τ=t) but still readable for a layperson; it also includes matches that are less likely to be in all fonts.
Replacing with: How much to replace?: