Sometimes you need unique identifiers in the output.

generate-id(node) will return the same unique identifier for a given node.

generate-id() just returns a value--your schema says if it is valid where you use it.

Example:

<xsl:template match="annotation">
<xsl:copy>
<xsl:attribute name="id"><xsl:value-of select="generate-id(.)"/></xsl:attribute>
</xsl:copy>
</xsl:template>