Massimo Ortolano is correct in saying that we use Meeks STV to attribute your vote to the candidates. The general overview is that you can rank as many of the candidates as you wish, in the order you prefer. Your first choice candidate will get your vote first and that vote will stay with them until one of two things happens:
- your candidate is eliminated
- your candidate reaches the threshold to be elected.
Should your top candidate be eliminated, your vote will then be transferred to the second-highest ranked candidate who remains in the election. Should the candidate reach the threshold, a fractional amount of your vote proportional to the overage being reallocated to other candidates will be awarded to your next-highest ranked candidate who remains in the election.
This process will continue until three candidates have reached the threshold.
Up until this very election, we only allowed voters to rank the top three candidate selections and this led to a potential for gaming the ranks - if you were certain that your favorite would win election, you might opt to rank them third - or not at all - which could cause a spoiler effect where your top candidate doesn't end up being elected at all. While unlikely, this is a known issue with all STV formats but by allowing you to rank all candidates rather than only three, we're hoping to reduce the effect since you will have less reason to not rank your top choice first.
The other caveat is that you should only rank candidates you wish to win the election do not rank a candidate who you feel is a bad choice for the position! If you dislike a candidate but vote for them anyway, there is a (unlikely) chance that your vote may actually go to that candidate - who you don't want to win.