Windows supports font face name substitution. Windows manages a list of substitutions; every substitution maps a logical font to an existing physical font.
For example, the substitution:
Helv=MS Sans Serif
maps theHelvetica font to the existing MS Sans Serif font.
maps MyFont (Western) to Arial (Western), and MyFont (Cyrillic) to Arial (Cyrillic).
The substitution actually is needed only when the substituted font currently is not loaded (is not installed). If the font is currently loaded, then the substitution is not used.
It is required to restart Windows to allow a new or changed font substitution to take effect.
Suppose your document is formatted with the font MyFont, and the language of this text is assigned to English (United States).
You open this document in WinWord.
Before WinWord shows the text, it asks Windows for the font that has the face name MyFont and the character set Western (=0). (The English alphabet is covered by the Western font character set.)
If MyFont font is loaded and it has the Western character set, then Windows provides exactly this font, and your text is successfully drawn on the screen.
If MyFont does not exist or MyFont font does not have the Western character set, then Windows looks for the substitution in the list of substitutions. If the substitution for MyFont, 0 exists, then Windows uses this substitution. If the substitution for MyFont, 0 does not exist, then Windows provides any existing font and the resulting font face may be arbitrary. In this case, the text drawn in WinWord may differ from what you expect.