Martin (1992: 218):
Like internal comparatives, internal additives are a richer resource internally than externally. This is in part as result [sic] of incorporating three of Halliday and Hasan's (1976) continuatives into the internal additive network (now, well, anyway); … Internal additives can be divided into those whose main function is to punctuate an exchange (oh and well) and those mainly concerned with building turns.
The cohesive system of continuity does not involve an expansion relation.
In SFL theory, addition is a type of extension, within the logical semantic relation of expansion. The subtypes of expansion are transphenomenal fractal types that are manifested across various domains at various scales. For example, elaboration, extension and enhancement are manifested in relational processes as intensive, possessive and circumstantial relations, respectively. When deployed textually as cohesive conjunction, the relation is marked grammatically by a conjunctive Adjunct.
Continuity, on the other hand, functions alongside cohesive conjunction, in dialogic texts, relating a clause in a new turn to a previous one (see Halliday & Matthiessen 2004: 534). It does not involve any type of expansion relation and is marked by continuatives, not conjunctive Adjuncts. It is a serious theoretical inconsistency to construe continuity as any type of expansion.