I'll give it another try....thanks for hanging in there with me:

****** Sample XML *******
<source id="VWX" description="This is VWX"/>
<source id="WXY" description="This is WXY"/>
<source id="XYZ" description="This is XYZ"/>
<reqt id="VWX-01" description="This is a requirement identified within
source VWX"/>
<reqt id="XYZ-01" description="This is a requirement identified within
source XYZ"/>
<reqt id="XYZ-02" description="This is another requirement identified
within source XYZ"/>
<package name = "pkg01">
  <class name="ABC">
    <source-assoc source="XYZ"/>
  </class>
  <class name="BCD">
    <reqt-assoc reqt="XYZ-01"/>
  </class>
  <class name="CDE">
    <source-assoc source="XYZ"/>
    <reqt-assoc reqt="XYZ-01"/>
    <reqt-assoc reqt="XYZ-02"/>
  </class>
  <class name="DEF">
    <reqt-assoc reqt="VWX-01"/>
  </class>
  <class name="EFG">
    <reqt-assoc reqt="XYZ-02"/>
  </class>
  <class name="FGH">
    <source-assoc source="WXY"/>
  </class>
</package>
*****************************


I want to count the number of classes that reference a set of sources,
e.g., "XYZ and VWX", via <source-assoc/> or <reqt-assoc/> tags
****** Desired output for XYZ and VWX *******
5
*****************************

My approach is to:
1) construct a variable with all source document id's concatenated and
delimited by '*', e.g.,
  <xsl:variable name="source-ids">*VWX*WXY*XYZ*</xsl:variable>

2) for each class, construct a variable by iterating through the
classes and adding a character, e.g., 'X',
   every time I find a one with an appropriate <source-assoc> or
<reqt-assoc/> tag, e.g.,
  <xsl:variable name="related-classes">
     <xsl:for-each select="package/class">
         <xsl:if test=".//source-assoc[contains($source-ids, '*' +
@source + '*')] | .//reqt-assoc[contains($source-ids, '*' +
substring-before(@reqt, '-') + '*')]">X</xsl:if>
     </xsl:for-each>
  </xsl:variable>
  NOTE: I want to include the delimiter in the contains function
parameter to ensure that the match is not based on a substring within a
source id
  NOTE: The <source-assoc/> and <reqt-assoc/> tags might actually also
be included within elements contained within classes.

3) display string-length($related-classes)

Problem: I don't believe "'*' + @source + '*'" is being processed
correctly within the contains function.

Any help would be appreciated!

Thanks in advance,
David