Halliday & Matthiessen (1999: 385):
An instantial system may fall entirely within the registerial system it instantiates; in other words, the meanings created within it may all have been created before. However, it may also create new meanings — new to the speaker and/or listener. In either case, the instantial system is built up successively by the generation process; but as it is developed, it in turn becomes a resource for further instantiation.