In the functions of language, the third, the descriptive one, takes as its reference to evaluate the effectiveness of the communications, the source. The communications is thus structurally one-way, and will end as the input data is fully processed.
On the contrary, the fourth function, the argumentative one, is goal driven, and evaluates effectiveness from feed-back. Its reference is the result, which is not known in advance, and depends on the recipient as much as on the source data.