Halliday & Matthiessen (1999: 179, 180):
Both types of element begin with a location in the 'here & now': they construct a path from the spatio-temporal moment defined instantially by the interaction base — the 'here & now' of the act of speaking — to a primary category of ideational phenomena: see Figure 5-2.
For example:
In other words, both types of group include deixis. But the deixis is of two different kinds: nominal deixis (such as near/remote) and verbal deixis (such as past/present/future), structurally realised as Deictic and Finite respectively. What this suggests is that, since processes occur in time — their mode of existence is temporal — that is how they are tied to the speech situation; whereas participants exist in some kind of referential space, which may be grounded concretely in the speech interaction (this = 'near me'; that = 'away from me') but may also be a more abstract, discoursal space. The latter is the space where we 'record' discourse referents as we work our way through a text (this = 'about to be mentioned (by me)'; that = 'mentioned earlier').