BATS Global Markets launched EDGX Options on Monday, the company's second options market, which has a different pricing model from the company's existing BZX Options market, designed to attract a different segment of options trading. Unlike BZX Options, which trades based on a price-time trading model with maker-taker pricing, EDGX Options will use a customer priority/pro rata allocation model, which means orders will trade based on price and size. "Two-thirds of U.S. options market volume is executed on exchanges with a pro rata model, and we believe we can help make markets better for participants in this segment of the market through our innovative technology, operating efficiency, market-leading pricing, and first-class customer service," said Brian Harkins, executive vice president and head of U.S. markets at BATS, when plans to launch EZX Options were first announced. Taken from: http://www.fiercefinanceit.com/story/new-bats-options-market-edgx-options-goes-live/2015-11-03
Pro rata to me means a match based on size. 10 people with orders matching the bid, 10 matches based on size. If you are 25% of the bid, you should get a 25% match. 100 trade, you get 25. I'm not sure if that is what they are doing. I'd have to look into it more closely or call them.