Halliday & Matthiessen (1999: 323):
The relationship which links the domain models with the general model is instantiation. Instantiation relates the system to the instance, at any given stratum. … A domain is located somewhere between the system and the instance. Hence it can be looked at from either end. Seen from the ‘system’ end, a domain is regular and repeated patterning in the way potential is deployed, over the course of time. Seen from the ‘instance’ end, it is generalisation across particular instances of text.