MS uses "custom XML". This refers to content within the file that is of a different XML format, with a separate "custom schema" to describe that content. The problem with such content is that there is no way for a standard to describe how such data should be interpreted, as it is by definition in a "custom format" and can be any kind of data. That is why "custom XML" is not allowed in ODF documents, and that is one of the reasons why OOXML is such a miserable standard.
sollte also MS in den naechsten dreisig Tagen keinen einen Aufschub erreichen, wird die lage misslich..