Halliday & Matthiessen (1999: 327):
It is important perhaps to make quite explicit the distinction among the three concepts of realisation, instantiation, and delicacy, since each of these is a distinct scale of abstraction. It is easiest to describe them in terms of metalanguage dynamics: what we are doing when we move along these different scales.(1) Realisation is the relation of one stratum to other strata (in any stratified system, with language as prototypical); when we shift attention from semantics '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 most general to most specific; and we can do it at any point along the instantiation scale, from system to text.
(2) Instantiation is the relation between the system and the instance. When we shift attention along this scale, we are moving between the potential that is embodied in any stratum and the deployment of that potential in instances on the same stratum. Again, this move can be made at any degree of delicacy.
(3) Delicacy is the relation between the most general features and the most specific. When we shift attention from, say, 'recreation' to 'hockey' at the level (stratum) of context, or from 'syllable' to long open nasal syllable' to /pã:/ in phonology, we are moving in delicacy. Again, we can do this at any point along the instantiation scale.