Section 19.2.2 says:

It is possible to apply validation to a document node. The circumstances
under which this happens are as follows:

    * A document is constructed using the xsl:document instruction, and the
validation or type attribute is specified explicitly or implicitly.
    * A result tree is constructed implicitly in the absence of the
xsl:result-document instruction, and the default-validation attribute of the
xsl:stylesheet element in the principal stylesheet module requests
validation.
    * A result tree is constructed using an xsl:result-document instruction,
and the validation or type attribute is specified explicitly or implicitly.
    * The xsl:copy instruction is used when the context item is a document
node, and the validation or type attribute is specified explicitly or
implicitly.
    * One or more of the items selected by the xsl:copy-of instruction is a
document node, and the validation or type attribute is specified explicitly
or implicitly.

I think the wording of this list dates from when we allowed
default-validation="strict" or default-validation="lax" to be specified on
the xsl:stylesheet element. This attribute now allows only the values
preserve and strip, which (as mentioned a couple of paragraphs later) do not
request validation.

I think the bullets should read:

    * A document is constructed using the xsl:document instruction, and the
validation or type attribute is specified.
    * A result tree is constructed using an xsl:result-document instruction,
and the validation or type attribute is specified.
    * The xsl:copy instruction is used when the context item is a document
node, and the validation or type attribute is specified.
    * One or more of the items selected by the xsl:copy-of instruction is a
document node, and the validation or type attribute is specified.

Or more concisely:

"It is possible to apply validation to a document node. This happens when a
new document node is constructed by one of the instructions xsl:document,
xsl:result-document, xsl:copy, or xsl:copy-of, and this instruction has a
type attribute or a validation attribute with the value strict or lax."

Previously raised at 
http://lists.w3.org/Archives/Member/w3c-xsl-wg/2005Oct/0043.html