Halliday (2002 : 168):
I shall assume the concept of register, or functional (diatypic) variation in language. It is convenient to talk of "a register", in the same way that one talks of "a dialect": in reality, of course, dialectal variation is typically continuous, along many dimensions (that is, with many features varying simultaneously), and what we call "a dialect" is a syndrome of variants that tend to co-occur. Those feature combinations that actually do occur — what we recognise as "the dialects of English", for example, or "the dialects of Italian" — are only a tiny fraction of the combinations that would be theoretically possible within the given language. Similarly, "a register" is a syndrome, or a cluster of associated variants; and again only a small fraction of the theoretically possible combinations will actually be found to occur.
Halliday M.A.K. 2002 Linguistic Studies of Text And Discourse London: Continuum