Originally posted by solvs
I found this, but to be honest I have no idea what it is. Don't have access to a Mac right now to test it.
Warning: taken from an xml doc (so pardon the code).
<li>Mac <span class="caps">OS X</span> has a Unicode font engine called <span class="caps">ATS,</span> and when you type a character that doesn't have an assigned glyph in the font you're using, <span class="caps">ATS</span> will go hunting through your fonts to find a font that <em>does</em> have a glyph assigned for that character. In the case where it cannot find <em>any</em> font with that glyph, it substitutes a glyph from the <a href="http://developer.apple.com/fonts/LastResortFont/">Last Resort Font</a>. If you look at the <a href="http://developer.apple.com/fonts/LastResortFont/LastResortTable.html">table</a>, you see that the glyph in the Last Resort Font for the Private Use Area (U+E000 through U+F8FF) is the character "wum" from Dr. Seuss's <cite>On Beyond Zebra</cite>. To easily generate it (the given answer at <span class="caps">WWDC</span> was tedious but correct), pull up TextEdit, enable and switch to the Unicode Hex Input input method, press and hold the Option key, type "EFFF" and release the Option key.</li>
</ol>
<p>As a bizarre aside, a project called the <a href="http://www.evertype.com/standards/csur/index.html">ConScript Unicode Registry</a> was established to assign Unicode code points in the Private Use Area to scripts that would never get an official assignment. And, fittingly enough, there is a block reserved for <a href="http://www.evertype.com/standards/csur/seuss.html">Seussian Latin Extensions</a>. So, it would probably be more appropriate to use U+E631 ("SEUSS <span class="caps">LETTER WUM</span>") for the example above.</p>