gentext: difference between uppper and lower case?

[Paul Tremblay <paulhtremblay@gmail.com>]

The customization of titles for the docbook xsl stylesheets requires that one use a key attribute for the following template:
    
<l:i18n xmlns:l="http://docbook.sourceforge.net/xmlns/l10n/1.0"> 
      <l:l10n language="en"> 
          <l:gentext key="glossary" text="Glossary Title"/>
      </l:l10n>
    </l:i18n>

However, it is not clear when one should use the upper case for the key, and one should use the lower case. The en.xml file as entries for both. In other words, why not use "Glossary" here (except the obvious that it won't work!).

An online search indicates that the mixture of lower and upper case results because of code from the DSSL that has been kept. 

Thanks

Paul