Suppose that I am typing text in MS Word (any version) and I enter a character that does not exist in the font being used. Say, I’m using Times New Roman and I type 2300 Alt X, which turns to the diameter sign “?”, which does not exist in Times New Roman. MS Word picks it up from a different font, like Arial Unicode MS. This may mess up the typographic style, or line spacing. And this happens without notice.

Perhaps the most inconvenient feature here is that MS Word does not automatically return to the original font. Subsequent text appears in the replacement font, unless the user sees what is happening and realizes that he needs to change the font.

The question is: Can such substitutions be controlled, e.g. by specifying the font(s) to be used as backup fonts? If not, is there any reliable documentation about it?