Martin (1992: 237):
A text with a simultaneous internal and external structure, including an implicit internal consequential is illustrated below.
[4:187] a. Ben was unlucky.b. He had to take steroids for his injured hamstringc. and then they introduced more sophisticated tests.
By the SFL definition of the term — see previous posts — there are no internal relations in this example.
In SFL theory, the logical relation in the clause complex (b. c.) is enhancement: temporal: later (rebranded as 'successive'), and the textual (cohesive conjunction) relation between the message (a.) and the message group (b. c.) is implicit enhancement: cause: reason (rebranded as 'consequential').
With regard to implicit conjunction, in general, Halliday & Matthiessen (2004: 549) recommend:
It is perhaps as well, therefore, to be cautious in assigning implicit conjunction in the interpretation of a text. It is likely that there will always be other forms of cohesion present, and that these are the main source of our intuition that there is a pattern of conjunctive relationships as well.