'R.O.N.', or 'Re-open Nominations', is a vote for 'None of the above candidates', where candidates are ranked in order of preference, from 1 (most preferable) onwards. So, should the result of an election where there are 2 posts available, be:
1st: John Smith, 450 votes
2nd: R.O.N, 100 votes
3rd: Joe Bloggs, 75 votes
Then John Smith and R.O.N are elected. Therefore nominations will re-open at a later date for a second election to fill the second position.
If Re-Open Nominations is elected to any position, then at the end of the count that position is declared vacant and nominations must later be re-opened for that position.