Sorting used for "Order by" is set to "ascending". But you can use <xsl:sort> in the tempate to ensure proper sorting.

For example, you've made a call of GetIPageXml function and used MenuTitle as the Order by field.

Now in your template you can change default sorting (ascending) to descending:

<xsl:for-each select="/in:inputs/in:result[@name='GetIPageXml']/IPage">
  <xsl:sort order="descending" select="@MenuTitle"/>
  <xsl:value-of select="@MenuTitle"/>
</xsl:for-each>