The <customquestions> element configures custom questions for your domain. It allows the author to specify a custom question by name rather than by url.
<customquestion name="string" location="string" type="enum"/>
customquestions / customquestionnameIdentifies the name of the custom question.

locationSpecifies the URL or resource name of the custom question.

typeBase type for the custom question. See Question interaction type.
See Also
Advanced Assessment Editor Custom Questions Domain Customization