Halliday & Matthiessen (1999: 129):
"Symbolic processing" is a generalisation across sensing and saying that foregrounds the fact that they can both project. But sensing and saying differ in the level of projection: sensing projects interior content, ideas; saying projects exterior content, locutions. The level of the projected content determines the typical status of the projected content: locutions may be either quoted or reported, with quoting being favoured in many types of discourse; in contrast ideas are typically reported and only rarely quoted. That is, ideas are construed as being further removed than locutions from experience that is shared.
Projection thus construes a distinction between interior symbolic processing (sensing) and exterior symbolic processing (saying). The distinction between 'interior' and 'exterior' is reinforced by the internal organisation of figures of sensing and saying.