L.1.18.1 Text Boxes

All VML-based drawing objects (except for connectors) support the addition of rich WordprocessingML content within their extents. When WordprocessingML contents have been added to a drawing object, the resulting text is contained within a text box.

When WordprocessingML content is contained within a text box, the text is represented within a single txbxContent element that contains all of the desired WordprocessingML content. Text box content cannot contain references to other document stories, nor can it contain other txbxContent elements. That is, nested shapes cannot have rich content.