No Preference
On Worldcon site selection ballot, the WSFS constitution requires that "No Preference" be one of the choices. Selecting "No Preference" indicates that the voter does not care which bidder wins. Ballots marked "No Preference" are effectively removed from the process and do not count for or against any candidate and do not counts towards the total number of ballots and thus not towards the number needed for a majority and thus a win.
The WSFS constitution allows non-natural-human entities (such as clubs and pet rocks) to cast site selection ballots, but requires that their ballots be cast as "no preference" so as to have no effect on the balloting process.
Since Worldcon site selection uses the preferential ballot is is possible for a voter to list "No Preference" in second or later place. (E.g., there are three bidders, A, B and C. The voter cares strongly for A, but, if A is eliminated, has no further preference. In that case, he may mark the ballot (1) A, (2) No Preference.)
No Preference, by its nature, can never be eliminated, so once a ballot is a No Preference ballot, it drops out of the count. Choices listed after No Preference on the ballot are never counted. There is no difference between voting No Preference and leaving the rest of the ballot blank.