These are ballots where a vote was tallied in the first round, but was not a part of the last round of counting. A ballot becomes inactive if: 

• The voter chose not to rank all candidates and all ranked candidates were eliminated
• Election administrators limited the number of rankings the voter could provide to a number less than the number of candidates listed on their ballot. This is called “involuntary exhaustion”
• The ballot contains an error that disqualifies it, like ranking multiple candidates as the first-choice. This is the most rare source of inactive ballots.