It is for the .doc format and other Office firmat until Microsoft Office 2008 included and do not provide a standard for the the post-2008 Office format (ending with the `x` in the file extension), AFAIK.
ODT (OpenDocument) is also XML-in-a-ZIP and is also an ISO standard (older than Microsoft's, in fact).
I don't think the older variants (.doc, .xls etc.) are standardized, but iirc Microsoft did eventually release some documentation on them.