Metaphors have structures and functions in language use, but as parts of utterances, they  can have a range of purposes in and for the discourse event they participate in. DMT depends on a great number of distinctions that are part of the encompassing theoretical framework, one of which has to do with the difference between language use and discourse event (this concerns the notion of level of cognition). Briefly, utterances function as the building blocks of discourse events, and metaphors do not just have functions at the level of language use but also at the level of discourse. Following Jonathan Charteris-Black, DMT calls discourse functions of metaphor their purpose.

The most important question for DMT is whether the distinction between deliberate and non-deliberate metaphor use has a direct relation with metaphor purposes. In brief, when metaphors are not deliberate, they do not emerge in the referential and communicative structure and functions of the utterance. If an utterance does not have metaphorical properties for these two utterance dimensions, it is hard to imagine that the metaphor may have a discourse purpose all the same. To rephrase this as a question: how can a non-deliberate metaphor that does not affect the mental representation of the utterance in the situation and context model still have a discourse function at the level of text in code in context that is mentally represented in one of the four mental representations? (For a well-know approach to analysing referential and communicative functions of utterances in discourse, see Rhetorical Structure Theory.)

This might perhaps only apply to the recognizability of the register of the text as a specific register because it comprises a characteristic number of non-deliberate metaphors, but the question is when this becomes psychologically or socially real, and to whom? Perhaps the issue of violence metaphors in cancer discourse offers an interesting testing ground. Similarly, when a domain of discourse promotes deliberate metaphor comprehension, such as literature, this might also lead to specific configurations of this kind.

Deliberate metaphors, on the other hand, come up in the situation and context model of an utterance and are therefore potential parts of the role of the utterance in the discourse. Many of the examples Charteris-Black provides for purposeful metaphors are also deliberate metaphors. DMT holds that this is not accidental. This is where metaphor exerts an overt function of framing.

One key issue for DMT, therefore, comprises the further exploration of the relation between deliberate and non-deliberate metaphor use, on the one hand, and the purposes of metaphor in discourse, on the other.