Halliday & Matthiessen (1999: 18):
Thirdly, there is the unfolding of the act of meaning itself: the instantial construction of meaning in the form of a text. This is a stochastic process in which the potential for creating meaning is continually modified in the light of what has gone before; certain options are restricted or disfavoured, while others are emprobabled or opened up. We refer to this as the logogenetic time frame, using logo(s) in its original sense of ‘discourse’.