The function of specifying the message type should be distinguished form another function of indicating the type of assessment expressed in the sentence. This can be made clear by postulating two aspects of language communication: the speech itself and the narrated matter. The narrated matter is the speaker’s view of the extra-linguistic universe, as he knows it to be existing or as he can envisage it to be possible to exist. The speech itself comes into play whenever he wants to use his linguistic competence to express his understanding of this universe. Any language should be able to carry out these two functions although how many types can be expressed and what they are would depend on each language.
What has been explain above with regard to typed of messages is related to speech itself. A sentence on the one hand has devices by which it can indicate what kind of communication is going on: whether passing on an information, eliciting an information or trying to influence others’ behavior, and so on. This function is however, quite distinct from that of giving an indication as to what sort of an assessment the speaker has in regard to the narrated matter, whether to him it is a verified thing or a doubtful one, whether he considers it as only a possibility or a certainty, etc. This latter kind of indication in a sentence, which has to do with the narrated matter as such could be termed modality.
On the content side, mood content or message type specification characterises the speech itself and modality characterises the narrated matter. One is outwardly directed, giving the sentence as a whole a certain character as to how it should be received; and the other is inwardly directed characterising the information contained in the sentence.