Halliday & Matthiessen (1999: 114):
The interpersonal system for dialogic interaction thus creates a fundamental distinction between ‘content’ as proposition and ‘content’ as proposal. This distinction is then reflected in the ideational system of projection: a figure is projected in one or other of these two modes, as a proposition or as a proposal. The two categories combine freely with ideas and locutions. Projected proposals are non-actual, or uninstantiated: that is, the occurrence of a projected proposal is always future in relation to the figure that projects it. In contrast, propositions are actual, or instantiated: that is, the occurrence of a proposition is located in actual time (which may be past, present or future).