what is the source for columns in xsl file PO form

Hi All,

We are modifying custom Standard Purchase Order  , Company Address update in XSL file.

<fo:block xsl:use-attribute-sets="legal_details_style">

    <xsl:value-of select="OU_NAME"/> <fo:block/>
    <xsl:if test="OU_ADDR1 !='' "> <xsl:value-of select="OU_ADDR1"/> <fo:block/> </xsl:if> 
    <xsl:if test="OU_TOWN_CITY !=''"> <xsl:value-of select="OU_TOWN_CITY"/>,
     <fo:leader leader-pattern="space" leader-length="2.0pt"/> </xsl:if> <xsl:value-of select="OU_REGION2"/><fo:leader leader-pattern="space" leader-length="3.0pt"/><xsl:value-of select="OU_POSTALCODE"/> <fo:block/>
    <xsl:value-of select="OU_COUNTRY"/>

</fo:block>

So what is the source for OU_NAME,OU_ADDR1 etc..