Halliday & Matthiessen (2014: 115-6):
An information unit does not correspond exactly to any other unit in the grammar. The nearest grammatical unit is in fact the clause; and we can regard this as the unmarked or default condition: other things being equal, one information unit will be co-extensive with one clause. But other things are often not equal, for reasons that will be brought out in the following sections. Thus a single clause may be mapped into two or more information units; or a single information unit into two or more clauses. Furthermore, the boundaries may overlap, with one information unit covering, say, one clause and half of the next. So, the information unit has to be set up as a constituent in its own right. At the same time, its relationship to the clausal constituents is by no means random, and instances of overlapping boundaries are clearly ‘marked’; so the two constituent structures, the clausal and the informational, are closely interconnected.