Global Elements and root element

Article 2.2.2 of XML Schema Part 0 Primer states:

"The declaration of a global element also enables the element to appear at
the top-level of an instance document.  Hence purchaseOrder, which is
declared as a global element in po.xsd, can appear as the top-level element
in po.xml.  Note that this rationale will also allow a comment element to
appear as the top-level element ina a document like po.xml."

This being the case, how do we programatically determine which should be the
root element of the instance document or do we make it a choice for the
user.

KK Aw