5 CFR §2422.28

1. (a)When a runoff may be held. A runoff election is required in an election involving at least three (3) choices, one of which is “no union” or “neither,” when no choice receives a majority of the valid ballots cast. However, a runoff may not be held until the FLRA has ruled on objections to the election and determinative challenged ballots.
2. (b)Eligibility. Employees who were eligible to vote in the original election and who are also eligible on the date of the runoff election may vote in the runoff election.
3. (c)Ballot. The ballot in the runoff election will provide for a selection between the two choices receiving the highest and second highest number of votes in the election.