Halliday & Matthiessen (1999: 407):
The speaker thus selects ‘thematic spaces’ as points of entry into larger regions of the ideational semantic network. From the listener’s point of view, these thematic spaces constitute indications of where to integrate the new information being presented in the text… . If we think of the listener’s processing of a text as being partly a matter of expanding his or her current semantic network with new information, the thematic spaces guide him/her to appropriate expansion points.