Ballot exhaustion occurs when a voter's ballot can no longer be counted, because all candidates supported on that ballot have been eliminated from an election. This may occur because the voter chooses not to fill out a complete preference ranking, or because the ballot format itself limits the number of preferences that may be expressed.
For example, under the Supplementary vote variant of Contingent vote, voters are limited to expressing their favorite and second favorite candidate. If neither candidate ranked by a voter on their ballot makes it to the runoff round, then that voter's ballot is exhausted and has no effect in the runoff round.
Similarly, the form of contingent vote used in Sri Lanka limits voters to expressing 3 rankings, so if there are more than 3 candidates, their ballots can be exhausted.
Such an outcome can also happen under instant-runoff voting, such as in Minneapolis, Minnesota. The city limits voters to 3 rankings of candidates on ballots for city elections.[1]