An extension attribute is an extra attribute on an XSLT-defined element. Following the rules of XSLT, such attributes must be in a non-default namespace. For Saxon extension elements, the namespace must be the Saxon namespace URI "http://saxon.sf.net/"
For example, the saxon:assignable
attribute can be set as follows:
<xsl:variable name="counter" saxon:assignable="yes"
xmlns:saxon="http://saxon.sf.net/">
The extension attributes provided with the Saxon product are as follows:
saxon:assignable: marks a variable as updateable
saxon:explain: requests display of optimized expression tree
saxon:memo-function: marks a function as a memo function