XPath and XQuery: fn:xxx

basically you will see the fn: used in books and web examples,
I never use them in my code
fn: indicates the namespace xmlns:fn="http://www.w3.org/2005/xpath-functions"
the namespace of the functions internaly defined in the XPath spec
but if you use the prefix, you also have to declare the prefix this way
xmlns:fn="http://www.w3.org/2005/xpath-functions"

eg.

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:fn="http://www.w3.org/2005/xpath-functions"
    exclude-result-prefixes="xs"
    version="2.0">
    <xsl:template match="/">
        <xsl:value-of select="fn:abs(5)"></xsl:value-of>
    </xsl:template>
</xsl:stylesheet>

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I simply use it without the prefix, for me that is much clearer

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    exclude-result-prefixes="xs"
    version="2.0">
    <xsl:template match="/">
        <xsl:value-of select="abs(5)"></xsl:value-of>
    </xsl:template>
</xsl:stylesheet>

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You can not declare functions yourself in that namespace, so your own functions will always need to be prefixed
so the distinction is easy
(I think an early XPath recommendation draft had the fn: as mandatory for XPath functionsn but the requirement was dropped)

Anyhow, simply forget about fn:, they are the same