Halliday & Matthiessen (2014: 635):
Sometimes an explicit indication may be given that something is omitted, by the use of a substitute form; for instance one in the following examples:||| He ran out on his wife and children, || became a merchant seaman, || was washed off a deck of a cargo ship || and miraculously picked up, not his own ship but another one, way out in the middle of nowhere. |||
||| ... so my decision was [[ that I should do three separate books, one on each generation]] . ||| – ||| What happened to the middle one? |||
||| ... if I am totally incapable of doing anything || or go into a stroke again || (the last one I had was on my right) || if I got a really whopping one || and could neither see || nor speak || – I would ask to be taken away. |||
The substitute is phonologically non-salient and serves as a place-holding device, showing where something has been omitted and what its grammatical function would be; thus one functions as Head in the nominal group and replaces the Thing (with which the Head is typically conflated).