OOXML is the XML schema used to create/construct DOCX documents. On https://en.wikipedia.org/wiki/Office_Open_XML you can read more information about OOXML.
Advanced and Premium licenses of phpdocx allows transforming DOCX to HTML. All licenses can transform HTML to DOCX; Premium licenses also include HTML Extended and CSS Extended. In all cases using OOXML that is used in DOCX documents. Although both features allow generating documents, you can also use them to generate only raw strings (OOXML and HTML) instead of documents.