DXRichEdit defines two types of structures for a document. First, the logical structure of a document, named Document Model, which reflects its logical hierarchical organization. Second, the physical structure, which represents the appearance of physical entities in a document. This is called the Document Layout, since it is a hierarchical description of a layout of a document.
When a document is processed by RichEditControl, the Document Model is initially built. Then the Layout Engine converts the Document Model into the Document Layout. So in general, there is no one to one correspondence between physical and logical entities.
The Document Layout is visualized by the Active View of RichEditControl.
The following articles describe the document structure in more detail: