Here is the current text for the Extensibility section of the Specification.
I've also included an example.
Note 1: XMLSpy doesn't validate correctly attributes in element where you
can have attributes from other namespace (i.e. doesn't flag errors). So for
example, testing attribute in <trans-unit> is not working, but testing in
<context> does, because <context> doesn't allow extra attributes.
Note 2: Look at the current list of pre-defined attributes by opening
xliff-values-1-1.xsd in a browser: the style sheet will display the values
in a more frendly way. Read the notes in yellow and the places where there
is '???' (they are unresolved issues).
Note 3: I'm not too comfortable with the way to extend values. It's working
this way, but I hope we can find a better mechanism. Maybe some help from
XSD specialist is needed.
Note 4: It's normal that the links in the excerpt won't work since the rest
of the document is cut off.
ps: FYI, I'll probably be slow with emails and not very productive next week