Fawcett (2010: 85):
Notice that while Halliday introduces the word "rank" in "Systemic theory", the idea that it means a 'rank scale' of units with the associated concept of 'accountability at all ranks' is simply omitted from what is presented as a summary of the "basic concepts" of the theory. One reason why this is particularly surprising is that the concept of the 'rank scale' is still reflected quite strongly in IFG, as we shall see in the next chapter. Nor is there any help on this matter in Halliday (1996). On the other hand, Matthiessen uses the concepts of the 'rank scale' and 'rank shift' quite freely in his Lexico-grammatical Cartography, and he defines 'rank' in the standard "Categories" manner in the book's useful Glossary section (Matthiessen 1995:790). Thus it would be premature to interpret Halliday's failure to foreground the concept of the 'rank scale' in "Systemic theory" as a weakening of his commitment to the 'rank scale'. It is nonetheless a curious omission.
Whatever the reason for this omission …
This is yet another attempt to mislead. What Halliday (1993: 273) actually says is:
3. Other Basic Concepts
Systemic theory retains the concepts of 'rank,' 'realisation,' and 'delicacy' from scale and category grammar. 'Rank' is constituency based on function, and hence 'flat,' with minimal layering;
Here Fawcett is attempting to cast doubt on the notion of rank, on the basis of the brevity of an entry in an encyclopædia entry. The reason Fawcett wishes to do so is that his own model does not include a rank scale, and so, by casting doubt, Fawcett primes the uncritical reader for his later dismissal of the theoretical value of a rank scale.