Halliday & Matthiessen (1999: 327):
Realisation is the relation of one stratum to other strata … when we shift attention ‘upwards’ into context or ‘downwards’ to lexicogrammar and phonology/graphology, we are moving in realisation. We can do this at any degree of delicacy, from the most general to the most specific; and we can do it at any point along the instantiation scale, from system to text.