Realisation and instantiation are clearly defined by being characterised in terms of the two types of relational processes.
(1) realisation is an [intensive] identifying relation:
the lower stratum (Token) realises the higher stratum (Value).
The difference is one of (symbolic) abstraction or signification.
eg coin as Token represents two dollars as Value.
These are two levels of abstraction of the same phenomenon.
(2) instantiation is an [intensive] attributive relation:
a text as Carrier (instance/member/specimen) of English language system as Attribute (class).
eg Tony Abbott as Carrier (instance/member/specimen) of Homo sapiens as Attribute (biological category).
Here’s a way to check usage:
If the relation being described crosses strata, then it is realisation.
Instantiation does not cross strata:
the system of semantics is ‘instantiated’ by the semantics of the text;
the system of lexicogrammar is ‘instantiated’ by the lexicogrammar of the text;
the system of phonology is ‘instantiated’ by the phonology of the text.
And at the level of context:
the system of context of culture is ‘instantiated’ by the context of situation.