preferential
/ˈpɹɛf.ɜː(ɹ)ˌɛn.ʃəl/
adjective
Vorzugsweise, bevorzugt
Of or relating to a voting system in which the voters are allowed to indicate on their ballots their preference (usually their first and second choices) between two or more candidates, so that if no candidate receives a majority of first choices the one receiving the greatest number of first and second choices together is the winner.
Vorzugswahlen; das Vorzugssystem