The overall architecture of SFL theory can be understood in terms of theoretical constructs within the model, namely: relational processes (identifying and attributive) and logico-semantic relations (expansion and projection).
‘Realise’ is an intensive identifying process, which means that it combines ‘identifying’ with ‘elaboration’.
In the case of strata, where a lower stratum realises a higher stratum, the relation between them is thus identifying + elaboration.
The rank scale, on the other hand, is organised in terms of composition, which is a subtype of extension. ‘Realise’ is used on the rank scale to relate the function of a higher rank to the form of a lower rank (eg Process is realised by verbal group). This relation between function and form is thus also identifying + elaboration.
The similarity thus lies in elaboration + identifying being an organising principle for both stratification and the rank scale.
The difference lies in the fact that, whereas stratification involves only elaboration + identifying, the rank scale combines extension (relation between forms) with elaboration + identifying (function–form relations).
(The term ‘realisation’ is used wherever there is an intensive identifying (token-value) relation in the theory. eg between system and structure on a given stratum, between strata, between function and form in the rank scale.)
Halliday & Matthiessen (1999: 4n):
The ambiguity [of ‘level’ as either stratum or rank] resides in the overlap of two grammatical relations, those of elaboration (‘be’) and of extension (‘have’) …