There may be situations in which MLTE cannot draw a character with the assigned font because the font is not installed on the user's system. If you set font substitution options, MLTE will use ATSUI's transient font matching function. The transient font matching function scans all valid fonts on the user's system until it finds a suitable substitute font. If you do not set this bit, missing character glyphs are used to represent unavailable characters.
You can use the font fallback mask described in "Text Box Options Masks" to set fallback options for static text display. If you want to use font substitution for a text object, you can use the font substitution tag described in "Formatting and Privileges Settings".