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 VML drawing object, the resulting text is contained within a text box.
When WordprocessingML content is contained within a text box, it is represented within the object by specifying the VML <textbox>
element, which contains within it 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.