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'Bullet voting' is a tactic where the voter only selects one candidate, despite having the option to indicate some preference for other candidates. They might do this either because it is easier than evaluating all the candidates, or (depending on the voting system in use) as a form of tactical voting.
If enough voters ''bullet vote'' then almost any voting system functions like plurality voting system. This is generally considered to be a poor result, as many voting systems are intended as reforms or improvements which avoid or minimize some of the disadvantages of plurality (aka First Past the Post). However, if voters bullet vote as a conscious strategy to express their meta-preference that candidates with strong support are preferable to "compromise" candidates with broad support, then it may not be a problem.
This tactic is possible in any voting system which does not require ranking all the candidates. In practice, this is the majority of voting systems as actually implemented (as opposed to in theory), since to reject ballots which don't rank all candidates would risk an excessive number of spoiled ballots. However, it is mainly an issue in voting systems which can reward this tactic — primarily approval voting and to a lesser extent range voting — and in those which don't significantly penalize it — primarily instant-runoff voting and a non-standard form of Borda count.
Candidates may seek to encourage bullet voting in certain situations. One example is where there is an election for two seats of the same office, and there are multiple candidates (say A, B, and C). Voters in such a situation typically have two votes. Candidate A will encourage his voters to vote only for him and not use their second vote. If the second vote is cast for B or C, it will help A's opponents. The situation is most pronounced where A is of one party and B and C are of another party. If voters from B and C's party vote for them, while A's partisans cast one vote for A and split their second vote between B and C, A will be significantly disadvantaged.