I was reading Robinhood's SEC Rule 606(a) disclosure report (Robinhood Securities - Held NMS Stocks and Options Order Routing Public Report) for Q1 2020. For orders on S&P 500 stocks in January 2020, 7.29% were market orders, 14.82% were marketable limit orders, 14.56% were non-marketable limit orders, and 63.33% were "other orders". 81.26% of "other orders" went to Citadel Execution Services.
I understand market and limit orders, but I don't know what these "other orders" are. I expected limit orders to be the most popular, followed by market orders. I am surprised that "other orders" actually accounted for the majority of orders. What are "other orders", and why are they more popular than market and limit orders?