...
<Form id="id24" name="160D999998-98/01" accessRefs="#id3" subType="Part Revision Master" subClass="Part Revision Master">
<Form id="id30" name="09A10-000019/A-AF Engineering Or" accessRefs="#id3" subType="AF Engineering Order E" subClass="AF Engineering Order E">
<Form id="id33" name="09A10-000019/A-AF Design/Draftin" accessRefs="#id3" subType="AF Design/Drafting Request A" subClass="AF Design/Drafting Request A">
<Form id="id36" name="09A10-000019/A" accessRefs="#id3" subType="EngChange Revision Master" subClass="EngChange Revision Master">
...


##</code><code>##

<xsl:value-of select=
 "/PLMXML/Form@subType='Part Revision Master']/UserData/UserValue[@title='cage_code']/@value"/>

<!-- also tried -->


	<xsl:strip-space elements="*"/>

<xsl:value-of select=
 "/PLMXML/Form@subType='PartRevisionMaster']/UserData/UserValue[@title='cage_code']/@value"/>

<!-- also tried -->

<xsl:value-of select=
 "/PLMXML/Form@subType='Part#x20Revision#x20Master']/UserData/UserValue[@title='cage_code']/@value"/>

<!-- also tried -->

<xsl:for-each select="PLMXML/Form">
  <xsl:variable name="formtype">
    <xsl:value-of select="substring(@subType,1,4)"/>
</xsl:variable>
  <xsl:if test="$formtype ='Part')">
                   <xsl:value-of select=
 "/PLMXML/Form/UserData/UserValue[@title='cage_code']/@value"/>
	</xsl:if>
</xsl:for-each>


<!-- also tried -->

<xsl:for-each select="PLMXML/Form">
  <xsl:variable name="formtype">
    <xsl:value-of select="substring"/>
  </xsl:variable>

	<xsl:value-of select="$formtype"/>
		<xsl:if test="contains($formtype,'Part')">
                   <xsl:value-of select=
 "/PLMXML/Form/UserData/UserValue[@title='cage_code']/@value"/>
	</xsl:if>
</xsl:for-each>