Original post by HeleniaAnon #33 explained it pretty clearly in their previous post (this thread is SO confusing with all the anons - come out people!)
To simplify it, imagine you have 3 hospitals A, B and C. Each one has 30 jobs to fill, and there are 90 prospective F1s. As has already been said, they look at the applicant with the highest score (call them applicant #1) and give them their first choice. Then they look at applicant #2 and if their first choice hasn't already been taken, give them that, and so on. If their first choice has been taken, they are offered whichever of their highest choices is still available. This continues right down to #90 - who will obviously get whichever job is left, but this might not be the last one on their list!
So say historically hospital A is the most popular, B next, and C least popular - so C's minimum score will be the lowest. But that can only ever give you historical data - people choose differently each time. So maybe you are applicant #80. You want hospital A/B/C in that order, but are not sure you "stand a chance" at A.
If you put them down in A/B/C order, and by the time the allocation gets to #80, A has filled 29 posts, B 28 and C 23, you will get hospital A. If A is full but B still has spaces, you'll get B. If they're both full, you'll get C. Not what you wanted, but at least you tried - and you were prepared for the possibility you might not get in by knowing the previous scores.
If, however, you don't think you stand a chance at A, so put C/B/A, even though you'd rather work at A, and by the time the allocation gets to #80, A has filled 29 posts, B 28 and C 23, you will still get C, because you put it first. If A is full, but B has spaces, you'd still get C, because you put it first. So you've ended up in a job you didn't especially want, and will never know if there was a chance to have got something better.
The most important thing in the way these allocations work is that by putting A/B/C, you don't decrease your chances of getting B compared with if you put B first as a "safer" choice. So there is no disadvantage to putting your genuine favourite first and ranking in true preference order, regardless of what your score is. Hope that makes sense?