Halliday & Matthiessen (1999: 510):
But there is one problem with this intellectual strategy: that the code for systems of this fourth type has not yet been cracked. There are two aspects to this problem. One is that we do not yet fully understand the nature of a linguistic fact: this is the problem of instantiation. The other is that we do not yet fully understand the nature of the relationship that is the semiotic analogue of the "cause : effect" of classical physics: this is the problem of realisation.
It is true that Saussure, and even more Hjelmslev, took important strides towards an understanding; but we are still arguing about what Saussure really meant (to us it seems that he had not clearly separated the two concepts of instantiation and realisation), and Hjelmslev has largely been ignored — Sydney Lamb (e.g., 1966a,b) is almost the only person who has tried to follow through his achievements. Probably it will be well into the next century before the picture comes to be clear.