Halliday & Matthiessen (2014: 473):
Here, one clause is presented as being in total or partial replacement of another. Variation falls into two subtypes — ‘replacive’ (‘instead’) and ‘subtractive’ (‘except’).
(a) Examples of clauses linked by the ‘replacive’ relation:
||| The vortex is not a uniform cylinder || but has a shape [[ that varies with altitude || and is strongest and most isolated above the 400-K isentropic surface, around 15 km and above]] . |||||| Witnesses said || the sand dredger seemed to go past the Marchioness || but suddenly smashed into the side || and went right over it. |||
||| They should not be broad statements [[ saying || where we hope to be]] , || but instead plans [[ specifying || what we want to do next || and exactly how we are going to do it]] . |||The clauses related in this way often differ in polarity value, one being ‘positive’ and theother ‘negative’. Note that the but here is not adversative, and so is not replaceable by yet; nor is it concessive — it does not correspond to hypotactic although. Cohesive expressions used with total replacement include instead, on the contrary.
(b) Examples of clauses linked by the ‘subtractive’ relation:||| He should have had them before, || only he hurt his shoulder at football or some such || and there was a long time spent in treatment, || so it was all deferred, || but finally he went. |||
||| Nelly looked rather put out || and replied || that he was quite all right, || only the poor little chap was highly strung. |||Here the secondary clause presents an exception to what has been said in the primary clause.