Halliday & Matthiessen (2004: 569):
… ellipsis is a relationship at the lexicogrammatical level: the meaning is ‘go back and retrieve the missing words’. Hence the missing words must be grammatically appropriate; and they can be inserted in place. This is not the case with reference, where, since the relationship is a semantic one, there is no grammatical constraint (the class of the reference item need not match that of what it presumes), and one cannot normally insert the presumed element. Reference, for the same reason, can reach back a long way in the text and extend over a long passage, whereas ellipsis is largely limited to the immediately preceding clause.