Halliday & Matthiessen (2014: 438):
All clauses linked by a logico-semantic relation are interdependent: that is the meaning of a relational structure — one unit is interdependent on another unit. Two clauses related as interdependent in a complex may be treated as being of equal status [paratactic] … each constitutes a proposition in its own right and could thus be tagged … They could in fact select for different moods …