Coordination is a grammatical term, for relations between grammatical units. Proposition is a discourse semantic term, for messages exchanging information.
 To be clear, in SFL theory, the traditional category of 'coordination' is interpreted as the logico-semantic relation of paratactic extension. Logico-semantic relations obtain at the level of semantics (meaning) and are realised at the level of lexicogrammar (wording).
 To be clear, in SFL theory, 'proposition' is 'the semantic function of a clause in the exchange of information' (Halliday & Matthiessen 2014: 139). The intrusion of the word 'discourse' reflects the fact that Martin (1992) rebranded Halliday's interpersonal semantic system as a 'discourse semantic' system. ('Message' is a semantic unit of the textual metafunction.)