<xsl:param name="var">Filed1='aaa' or Field1='bbb'</xsl:param>

<xsl:for-each select=".">
<xsl:if  test=[$var]">
 ..........
</xsl:if>
</xsl:for-each>

<xsl:param name="var">|aaa|fff|bbb|</xsl:param>
...........................
<xsl:for-each ......>
<xsl:if test="contains($var, concat('|', BSVALUE/text(), '|'))">
...............
</xsl:for-each>