Halliday & Matthiessen (1999: 324):
… a semantic space in which meanings are constantly being realigned and new meanings created. In other words, we are describing a register; or, more accurately, a region of registerial variation (sometimes called 'diatypic' variation to suggest the analogy with dialectal variation). Register is systematic variation within the overall meaning potential …
The overall semantic potential is diversified into registerial varieties that emerge as patterns of instantiation.