UFC 31 was the first UFC event to use the modern weight-class structure.
Held on May 4, 2001, in Atlantic City, New Jersey, UFC 31 used weight limits that closely matched the divisions later standardized under the Unified Rules of Mixed Martial Arts. The event included heavyweight, light heavyweight, middleweight, welterweight, and lightweight bouts.
The change followed the UFC’s adoption of the Unified Rules, which replaced the looser tournament-era format with regulated divisions and commissions. Weight classes made matchmaking more consistent and helped distinguish professional MMA from its earlier no-holds-barred image.
UFC 31 is therefore more specific than UFC 28, the first event held under the Unified Rules. UFC 28 introduced the rules, while UFC 31 is generally identified with the modern division structure.