Halliday (2008: 97):
A virtual entity such as heat resistance is construed logogenetically: it is built up in the course of the text — which may be a single text or, more probably, the macrotext of some scientific subdiscipline. (Virtual processes, on the other hand, like cause and inhibit, tend to be general across all registers of learned discourse.) Such an entity is quite likely to become systemic: it enters the system and becomes a “technical term”. Other nominalisations may remain purely instantial: they function at that moment in the text, but are created each time out of the general resources of the grammar.