There is much disagreement about the units and elementary relations of discourse structure, but they agree that the structures are hierarchical, most commonly trees (Marcu, 2000), while others have argued for directed acyclic graphs (Danlos, 2004), or general graphs (Wolf and Gibson, 2004).
most of the segmentation research to date has focused on linear segmentation, in which segments are non-overlapping and sequential, and it has been argued that this sequence model is sufficient for many purposes (Hearst, 1994).