Mick O'Donnell wrote on 31 October 2014 to sysfling:
It all comes down to which criteria you put as primary in defining process types: the notional or the grammatical. You [David Banks] are saying meaning is first (processes of communication) and grammar (projection) second.
Others put grammar first, dealing with verbs which project on one side, then splitting these into verbal and mental subsets.
I argue elsewhere that the lack of clarity as to the priority between the notional and grammatical criteria is the reason behind much of the different classification decisions made within our community.
 Transitivity criteria do not boil down to the opposition between 'notional' and 'grammatical'; the relevant theoretical dimension is stratification. As already clarified here — where O'Donnell advocates following the 'grammatical principles that Halliday established' — SFL was theorised by taking a trinocular perspective, which means also looking at the grammar 'from above' (what meaning is being realised) and 'from below' (how the wording is realised) — as well as 'from roundabout' (the level of grammar). As Halliday & Matthiessen (2004: 31) make clear:
We cannot expect to understand the grammar just by looking at it from its own level; we also look into it ‘from above’ and ‘from below’, taking a trinocular perspective. But since the view from these different angles is often conflicting, the description will inevitably be a form of compromise.
 A functional grammar gives priority to the 'view from above' — that's what makes it functional rather than formal, since function (Value) is realised by form (Token). As Halliday & Matthiessen (ibid.) clarify:
Being a ‘functional grammar’ means that priority is given to the view ‘from above’; that is, grammar is seen as a resource for making meaning — it is a ‘semanticky’ kind of grammar. But the focus of attention is still on the grammar itself.
The motivation for this priority is explained by Halliday & Matthiessen (1999: 6) as follows:
But to show that a grammar is a theory of experience we use a functional, semantically motivated grammatics, since this allows us to seek explanations of the form of the grammar in terms of the functions to which language is adapted.