This distinction is often made in connection with two purposes (or functions) of language use. To use language is to attempt to achieve both at some level, with more or less interpretation needed to discern or explain these functions. Similar distinctions such as representative/expressive (Buhler), referential/emotive (Jakobson), and ideational/interpersonal (Halliday) follow a similar path.
The first one then is the transactional function: we (i.e. humans) use language to send messages with content, to send, that is, a representation of a non-linguistic content into a linguistic form. In the history of linguistics nobody was narrow-minded enough to claim that this is the only function of language, but often analysts have abstracted away from other functions in order to study what is “delivered” from one party to the other. The term “propositional content” is often used by semanticists who wish to state in very precise terms what a proposition communicates, regardless, as it were, from where and why and by whom it is uttered. It is assumed, as it where, that what the speaker primarily has in mind is a transfer of information.
The second function is the interactional function: we (i.e. humans) use language to establish and maintain various sorts of social relationships. Sociologists and anthropologists often speak of the phatic function of language and are keen to point that some of our everyday talk can even be said to have this function above the first one. As Brown & Yule (1983) notice that
“when two strangers are standing shivering at a bus stop in an icy wind and one turns to the other and says ‘My goodness, it’s cold’ it is difficult to suppose that the primary intention of the speaker is to convey information. It seems much more reasonable to suggest that the speaker is indicating a readiness to be friendly.”