Read a variable value in xslt

I am facing issue in reading the value of a variable from Header tag into LineDetail tag. I need to do conditional mapping based on this variable value. Could anyone please help me in resolving this?

Below is the sample of my transformation rule and I need to read the value of variable $varPOFlag in LineDetail tag.

    <?xml version="1.0" encoding="UTF-8" standalone="no"?>
    <xsl:stylesheet xmlns:xsl="http://ift.tt/tCZ8VR" xmlns:msxsl="urn:schemas-microsoft-com:xslt" version="1.0" exclude-result-prefixes="msxsl">
        <xsl:output method="xml" indent="yes" />
        <xsl:template match="Invoice">                    
                <Invoice>         
                <Header>
                        <!-- Read the PONumber if exists  -->
                        <xsl:variable name="varHeaderPONumber">
                                <xsl:choose>
                                <xsl:when test="./Header/PONumber != ''">                                         
                                        <xsl:value-of select="./Header/PONumber" />
                                </xsl:when>
                                </xsl:choose>
                        </xsl:variable>   
                        <!-- If PONumber does not exists in <Header> check at each lineItem level  -->
                        <xsl:variable name="varLinePONumber">     
                                <xsl:choose>
                                <xsl:when test="not(./Header/PONumber)" >
                                        <xsl:for-each select="./LineDetail">                                              
                                                <xsl:if test="position()=1">                                                              
                                                        <xsl:value-of select="./PONumber" />                                                                                                                      
                                                </xsl:if>                                                     
                                        </xsl:for-each>   
                                </xsl:when>
                                </xsl:choose>                                     
                        </xsl:variable>                           
                        <!--check if <Header.PONumber> matches with each <LineDetail.PONumber> -->
                        <xsl:variable name ="var1212">
                                <xsl:choose>
                                <xsl:when test="$varHeaderPONumber != ''">                                                                                                        
                                <xsl:for-each select = "./LineDetail">                                                            
                                        <xsl:choose>
                                        <xsl:when test="./PONumber !=''" >                                                                                
                                                <xsl:choose>
                                                <xsl:when test= "$varHeaderPONumber !=./PONumber">
                                                        <xsl:value-of select="'DoesNotMatch'"/>
                                                </xsl:when>                                                                                       
                                                </xsl:choose>                                                                     
                                        </xsl:when>                                                                       
                                        </xsl:choose>                                                     
                                </xsl:for-each> 
                                </xsl:when>       
                                </xsl:choose>
                        </xsl:variable>   
                        <!-- set the varPOFlag = 0 /1 if PONumber exists / does not exist -->
                        <xsl:variable name="varPOFlag">
                                <xsl:choose>
                                <xsl:when test="$varHeaderPONumber!='' or $varLinePONumber != ''">
                                        <xsl:value-of select='1'/>
                                </xsl:when>
                                <xsl:otherwise>
                                        <xsl:value-of select='0'/>
                                </xsl:otherwise>
                                </xsl:choose>                             
                        </xsl:variable>                           
                </Header>
                <LineDetail>
                                <!--Here I Need to read the value of variable $varPOFlag -->
                                <xsl:choose>
                                <xsl:when test="$varPOFlag=0">
                                        <xsl:value-of select="'Number'"/>
                                </xsl:when>
                                <xsl:otherwise>
                                        <xsl:value-of select="'Description'"/>
                                </xsl:otherwise>
                                </xsl:choose>
                </LineDetail>
                </Invoice>
        </xsl:template>
    </xsl:stylesheet>

Thanks in advance.