Book: Describing Language: Form and Function
Chapter: Chapter 5: Choice, System and Realization: Describing Language as Meaning Potential 
Reference to “choice” in SFL theory has no concern with individuality, freedom, will or all those other qualities that dictionaries assign to the act of choice. Rather “choice” is being used metaphorically to explicate the inner workings of language. Choice, system and realization together represent one critical aspect of the semiotic mechanism whereby the linguistic sign comes to have some conventionally shared meaning in speech fellowships.