The remainder of the clause we shall call the Residue. It has sometimes been labelled ʻPropositionʼ, but this term is also not very appropriate; partly because, as has been mentioned, the concept of proposition applies only to the exchange of information, not to the exchange of goods-&-services, and partly because, even in the exchange of information, if anything it is the Mood element that embodies the proposition rather than the remainder of the clause.
- Halliday (1985), a pag.74

In a proposition, the meaning of the positive and negative poles is asserting and denying: positive ʻit is soʼ, negative ʻit isn’t soʼ. There are two kinds of intermediate possibilities: (i) degrees of probability: ʻpossibly / probably / certainlyʼ; (ii) degrees of usuality: ʻsometimes / usually / alwaysʼ. The former are equivalent to ʻeither yes or noʼ, i.e. maybe yes, maybe no, with different degrees of likelihood attached. The latter are equivalent to ʻboth yes and noʼ, i.e. sometimes yes, sometimes no, with different degrees of oftenness attached. It is these scales of probability and usuality to which the term ʻmodalityʼ strictly belongs.
- Halliday (1985), a pag.86