It is very simple: you would use <xsl:processing-instruction>it if you needed to output the processing instruction to your XML output. Unless you need a PI in your output, you do not need an instruction processing element.
, PI , , .
, <?xml-stylesheet> :
<xsl:processing-instruction name='xml-stylesheet'>
type="text/xsl"
href="<xsl:value-of select='$stylesheet'/>"
media="screen"
</xsl:processing-instruction>
:
<?xml-stylesheet type="text/xsl" href="http://nedbatchelder.com/rss.xslt" media="screen"?>