1. Think of a system network, such as that of TRANSITIVITY (IFG3 p302). Think of it as coloured black.
2. Now, for example, think of a clause.
3. Now colour green all the features and realisation statements that are selected for that clause.
The term ’system’ refers to the entire TRANSITIVITY network.
The term ‘instance’ refers to just the green bits.
The term ‘instantiation as process’ refers to the process of applying the colour green.
The term ‘instantiation as scale’ — ‘the cline of instantiation’ — refers to the relation between the entire system and the green bits.
The green bits represent both a subpotential of the system, and the “activation” of that subpotential.
The instance is the “activated” portion of the system. The relation of the instance to the system is the relation of the “activated” portion to the system as a whole.
Halliday & Matthiessen (1999: 45):
The instance is thus a set of features selected, with associated realisational specifications — an instantial pattern over the potential.