<xsl:template match="sources">
    <xsl:element name="dataloggerlist">
        <xsl:for-each select="source">
            <xsl:apply-templates select="/pfarr/pfarr/pfarr" />
        </xsl:for-each>
    </xsl:element>
</xsl:template>

"sources" never matches anything. This template does nothing (or your input contains more).

summary.xml:
<sources>
    <source href="file1.xml" />
    <source href="file2.xml" />
    <source href="file3.xml" />
    <source href="file4.xml" />
</sources>

The triple path is not needed, because you already select this same path. What you are saying to the XSLT processor here is: any "pfarr" (most right in the match) that has a parent "pfarr" (middle) that has a parent "pfarr". Since you already do a select="pfarr/pfarr/pfarr", which only selects the 'pfarr' children of the 'pfarr' children of 'pfarr', there is no need to let the processor do this extra parent check.

<xsl:sort select="param/@id{dlt}" data-type="number" order="descending" />
<xsl:sort select="@name" order="ascending" />

I see now that you want to:

1. put all elements with id = 'dlt' first
2. order these first elements by number value
3. next, order all other elements, not containing 'dlt' by there id

<pfarr name="A">
	<pfstring id="dlt">20</pfstring>
</pfarr>

<pfarr name="B">
	<pfstring id="dlt">80</pfstring>
</pfarr>

<pfarr name="C">
	<pfstring id="dlt">50</pfstring>
</pfarr>

<pfarr name="D">
	<pfstring id="dlt">50</pfstring>
</pfarr>

<datalogger name="B">
	<param id="dlt">80</param>
</datalogger>

<datalogger name="C">
	<param id="dlt">50</param>
</datalogger>

<datalogger name="D">
	<param id="dlt">50</param>
</datalogger>

<datalogger name="A">
	<param id="dlt">20</param>
</datalogger>

This looks like this, in XSLT (but is not trivial)

<xsl:sort select="@name = 'dlt'" order="descending"/>
<xsl:sort select="self::node()[@name = 'dlt']" data- type="number" order="ascending"/>
<xsl:sort select="@name" />

Finally, this is the complete stylesheet that does what you want:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/ Transform">
<xsl:output indent="yes" />
<xsl:template match="/">
<xsl:apply-templates select="pfarr/pfarr/pfarr" />

</xsl:template>
<xsl:template match="pfarr">
<datalogger name="{@name}">
<xsl:apply-templates select="pfstring">
<xsl:sort select="@name = 'dlt'" order="descending"/>
<xsl:sort select="self::node()[@name = 'dlt']" data- type="number" order="ascending"/>
<xsl:sort select="@name" />
</xsl:apply-templates>
</datalogger>
</xsl:template>

  <xsl:template match="pfstring">
      <param id="{@name}" >
          <xsl:value-of select="." />
      </param>
  </xsl:template>
</xsl:stylesheet>

Sincerely,
- Rob