How does the pro-rata matching method differ from price/time priority in trading?

The pro-rata matching method indeed focuses on the amount of contracts available when matching orders. In this system, trades are executed based on the proportion of the total volume of orders rather than their price or the time they were received. This means that when a new order enters the market, it is filled based on how much volume it offers relative to other orders at the same price level.

In contrast, price/time priority prioritizes orders based on their price first, and then by the order in which they were received if there are multiple orders at the same price. This method ensures that investors who place their orders at better prices are executed before those at worse prices, and if prices are the same, the earlier order gets executed first.

While other options touch on aspects related to the differences between the two methods, they do not effectively encapsulate the core distinguishing feature of pro-rata, which is its emphasis on volume rather than price or timing. Thus, focusing on the amount of contracts perfectly captures how pro-rata differs from price/time priority.