Halliday & Matthiessen (2014: 148):
(i) From below, it is that nominal element (nominal group or nominalised phrase or clause) that is picked up by the pronoun in the mood tag.
(ii) From round about, it is that which combines with the Finite (operator) to form the Mood element in the clause; it is also that which constitutes the unmarked Theme if the mood is declarative, and which switches place with the Finite if the mood is yes/no interrogative.
(iii) From above, it is that which carries the modal responsibility; that is, responsibility for the validity of what is being predicated (stated, questioned, commanded or offered) in the clause.
This last point is the basic insight that informed the original, pre-structuralist interpretation of the Subject function, that in terms of a configuration of Subject + Predicate.