In Word Options > Advanced > Show document content, there is a "font substitution" button.

When I click that, my Word 2013 says "Book Antiqua Bold" is a missing document font, and that it is substituting "default". This is a misleading dialog, since my Word 2013 does in fact use "Book Antiqua Bold" (both in the UI and its PDF output).

Compare my Word 2016, which says "Book Antiqua Bold" is a missing document font, but that it is substituting "Book Antiqua", which it will display and use.

So in both cases it says the bold variant of the font is missing (even though it is in Windows\Fonts); what differs between Word versions is what it does about it!

I guess this same behaviour happens with any font in the font dir that has "bold" or italic or bold/italic styles. Is this behaviour documented anywhere?