Halliday & Matthiessen (2004: 384):
A multivariate structure is a configuration of different functional relationships … . Note that, although it is the functions that are labelled, the structure actually consists of the relationships among them.
Clause Complex: Univariate
Halliday & Matthiessen (2004: 383-4):
The tactic structures of complexes are relational in nature; they are the kind of structure that we have called univariate, to distinguish it from the multivariate structures … . A univariate structure is an iteration of the same functional relationship; … Such iterative structures are unique to the logical mode of meaning; they are formed out of logico-semantic relations.