"Tiebreaker rules" are a set of guidelines or procedures used to break a tie or determine a winner when two or more participants have the same score or result in a competition or event. Full definition
For situations where the same child may be eligible to be claimed as a dependent or qualifying child by more than one person, the IRS has established a set of tiebreaker rules to determine who has the right to claim the tax benefits. (efile.com)
What are the official tiebreaker rules in that instance? (sbnation.com)
In this case — or if it's really close and the two of you can't decide which parent your child stayed with more — the IRS uses what it calls «tiebreaker rules for claiming dependents» for sorting things out and deciding which parent is eligible to claim the child as a dependent. (thespruce.com)