The xsl:copy-of element copies the value obtained by evaluating the mandatory select attribute. It makes an exact copy.

If this expression is a string, a number, or a boolean, the effect is the same as using xsl:sequence.

There is an optional attribute copy-namespaces whose value is "yes" or "no". The default is "yes". This controls whether the in-scope namespaces of any element nodes copied by this instruction are automatically copied to the result. If the value is "no", then namespaces will only be copied if they are actually used in the names of elements or attributes. This allows you, for example, to copy the contents of a SOAP message without copying the namespaces declared in its envelope.

Making a deep copy of an element is particularly useful during streaming. For this reason XSLT 3.0 introduces an equivalent function, copy-of. For example one can write <xsl:for-each select="employee/copy-of()"> which processes a sequence of copies of the selected employee elements; the significance of using copies is that streaming constraints do not apply, since each employee element will be represented as a tree in memory, discarded as soon as that element has been processed.