https://lmarena.ai/?leaderboard

By arena score, when it first appears on the leaderboard (unless it doesn't appear there in the first month after release, in which case the market resolves N/A).

If they announce two models at once (for example, they release 3.8 opus and 3.8 sonnet at the same time or on the same day), then this market will consider the highest arena score of the models announced on that day.