What Is a “Pro-Rata” Matching System and How Does It Differ from Price-Time Priority?

A pro-rata matching system fills orders at the same best price proportionally to their size, rather than by the time they were submitted. If multiple orders are at the best price, a pro-rata system would fill a trader with a 100-lot order ten times more than a trader with a 10-lot order.

This differs from price-time priority, which would fill the oldest order first, regardless of size. Pro-rata is often used in futures and derivatives markets.