Halliday & Matthiessen (1999: 18):
These are the three major processes of semohistory, by which meanings at continually created, transmitted, recreated, extended and changed Each one provides the environment within which the 'next' takes place, in the order in which we have presented them; and, conversely, each one provides the material out of which the previous one is constructed: see Figure 1-6.
As the upward pointing arrow suggests, the individual's (transfinite) meaning potential is constructed out of (finite) instances of text; the (transfinite) meaning potential of the species is constructed out of (finite) instances of individual 'meaners'.
Following the downward arrow, the system of the language (the meaning potential of the species) provides the environment in which the individual's meaning emerges; the meaning potential of the individual provides the environment within which the meaning of the text emerges.
The sense in which grammar is said to construe experience will be somewhat different in each of these three time frames.