Martin (1992: 203):
The external comparison network is a small one; the basic opposition is between [contrast] and [similarity]:
CONTRAST[4:95] Whereas usually we winthis time we lost.
SIMILARITY[4:96] As usually happenswe won.
 This confuses the notion of contrast (A in contrast to B) with the notion of dissimilarity (A is unlike B).
 In SFL theory, the logical relation in [4:95] is extension: addition: adversative ('but'). Its meaning is X and conversely Y: usually we win and conversely (but) this time we lost. (The logical relation in [4:96] is manner: comparison — N is like M).
The basic opposition that is proposed for 'external comparison' (enhancement) is thus, in SFL terms, between adversative addition (extension) and manner: comparison (enhancement).