Halliday (2008: 114):
Whereas dialect variation was variation according to the user, register variation was variation according to the use. Like dialect, register was the name of a kind of variation; so if we talk of “a register”, this is again analogous to “a dialect”, in the sense that each refers to a clustering of features that typically go together. In the case of register, these will be explained in terms of the view from above — the context of situation [i.e. context as instance] and the context of culture [i.e. context as system]. In other words, a pattern of regularities in this middle range of generality (pertaining to less than the language but more than the individual text) is most clearly observed from the “system” position, where it will appear as a motivated resetting of the global probabilities of the language.