While the XSLT 1.1 standard was never finalized, libxslt adopted some parts of it anyway, including the <xsl:document> instruction element, which later came to be known as <xsl:result-document> in XSLT 2.0. This element is used to output XML, HTML, or text to an output file.
Internal to libxslt (and therefore, to Junos OS), it shares code with four other extension elements:
<exsl:document>, <redirect:write>, <saxon:output>, and <xt:document>
So when we added append support to <redirect:write> in Junos OS Release 11.1, we actually added it to all five elements. I would recommend that script writers use <xsl:document> rather than <redirect:write> because there is no need to include an extension namespace for XSLT elements.
Read more in Excerpt on the <xsl:document> Instruction Element