Halliday & Matthiessen (1999: 23-4):
This process of codifying may take place at any point along the cline from grammar to lexis, from grammaticalisation at one end (cf. Hopper & Traugott, 1993) to lexicalisation at the other. Perhaps the most highly coded meanings are those which are fully grammaticalised: that is, organised into grammatical systems, such as tense and polarity in English. This does not mean that they must be overtly signalled in syntax or morphology; some of them are, but others are uncovered only through systematic analysis, such as the different types of process configurations in English.
Lexicalisation may take the form of the instantaneous creation of new lexicalised meanings; like sputnik in 1958 or gazumping sometime in the seventies. But more often it is the end point of a process of lexical compacting, as in the example of quadruped above. Since lexicalised meanings do not form clearly defined and bounded systems in the way that grammaticalised ones do, we might consider meanings of this kind less highly codified, although the process of codification is the same in both cases.
Somewhere between the two extremes of grammar and lexis we may recognise the emergence of distinct grammatical structures and lexical classes. In the course of the history of English the meaning 'it is precipitating' became highly codified, in that types of precipitation came to be lexicalised as verbs (rain, hail snow, sleet, thunder, lighten) in a unique class having no participants associated with it e.g., it's raining, where the it functions as Subject but has no role in transitivity. (Note humorous back-formation on model of Actor-Process: What's it doing ?—Raining.)