The tone group [...] is not only a phonological constituent; it also functions as the realization of something else, namely a quantum or unit of information in the discourse. Spoken discourse takes the form of a sequence of INFORMATION UNITS, one following the other in unbroken succession with no pause or discontinuity between them.
- Halliday (1985), a pag.274

The information unit is what its name implies: a unit of information. Information, as this term is being used here, is a process of interaction between what is already known or predictable and what is new or unpredictable [...] Hence the information unit is a structure made up of two functions, the New and the Given.
- Halliday (1985), a pag.274-275

Structurally, therefore, we shall say that an information unit consists of an obligatory New element plus an optional Given.
- Halliday (1985), a pag.275

Besides being the domain of the organization of the information process into Given and New, through the location of the information focus, the constituent that we call the ʻinformation unitʼ has another function, an interpersonal as distinct from a textual one. It is the domain of the choice of TONE.
- Halliday (1985), a pag.281