Halliday & Matthiessen (2014: 115):
[information] is a system not of the clause, but of a separate grammatical unit, the information unit. The information unit is a unit that is parallel to the clause and the other units belonging to the same rank scale as the clause.
- clause
- group/phrase
- word
- morpheme
Since it is parallel with the clause (and the units the clause consists of), it is variable in extent in relation to the clause and may extend over more than one clause or less than one clause; but in the unmarked case it is co-extensive with the clause.