Make the default XPath version implementation defined

xproc/src/main/xml/specification.xml

@@ -6352,10 +6352,17 @@ If the attribute is not specified, the value
 <listitem>
 <para>The requested <tag class="attribute">xpath-version</tag>
 <rfc2119>must</rfc2119> be used to evaluate XPath expressions subject
-to the constraints outlined in <xref linkend="xpath-context"/>. If the
-attribute is not specified, the value “<literal>3.1</literal>” is
-assumed. <error code="S0110">It is a <glossterm>static error</glossterm> if
+to the constraints outlined in <xref linkend="xpath-context"/>.
+<error code="S0110">It is a <glossterm>static error</glossterm> if
 the requested XPath version is less than “<literal>3.1</literal>”</error>.</para>
+<para><impl>If a pipeline does not request a specific XPath version, the version
+used is <glossterm>implementation-defined</glossterm>.</impl> <impl>If different
+pipelines or libraries declare different XPath versions, it is
+<glossterm>implementation-defined</glossterm> how those conflicts are resolved.</impl>
+An implementation might use different versions for different pipelines, or it might
+use the same version for all pipelines. <impl>If an implementation elects to us the
+same version for all pipelines, the version selected is <glossterm>implementation-defined</glossterm>.
+</impl></para>
 </listitem>
 </varlistentry>
 <varlistentry><term><tag class="attribute">exclude-inline-prefixes</tag></term>