XML tools enforce XML syntax. XML syntax is very similar to HTML syntax, except,
like XHTML, the syntax is strictly enforced. The syntax is:
Tags are enclosed within angle brackets, for example, <para>
Opening tags must be paired with closing tags, for example,
<para>Sentence</para>
Opening and closing tags must be nested correctly, for example, the
following example is well formed
<note><para>Sentence</para></note>
The following example is not well formed
<note><para>Sentence</note></para>
If these syntax rules are followed, the XML document is said to be
well formed. That does not mean, however, that the tags in the XML document are
necessarily the ones defined in the associated WSDL. If, for example, the WSDL
defines <para>, an XML document that uses <Para> would cause an error
because "Para" is not defined in the WSDL. When an XML document is well
formed and the tags it uses conform to tags defined in the WSDL, the document is
said to be valid. Product Advertising API responses always contain valid
XML.