I am guessing that you are talking about the implementation side (on the webpage) of Test and Target?
If this is the case, a "class", Which in Test and Target is named "mboxDefault" is nothing more than a HTML class attribute identifier for the mboxCreate("mboxName") Test and Target javascript function which is available in the mbox.js.
I have not used the ID attribute in any implementation but I do know that the class is mandatory and the ID is not.
The used default implementation in all cases would be:
<div class="mboxDefault"> ... content that you want to test ... </div>
<script type="text/javascript">mboxDefault("mboxName");</script>
The listening mbox can either be a regular mbox or a conversion mbox. The mbox or "marketing box" is a piece of technology which will replace the content on the page with the content that you have defined in your offers within Test and Target.
If you have any questions don't hesitate to ask. Good luck with Adobe Test and Target!