What are "Other Orders" in order routing reports?

Question

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?

Answer 1

My guess for why "other orders" accounted for the majority of orders is that these are conditional orders such as stop loss, stop limit, AON, GTC, trailing stop, as well as some less frequent ones like fill-or-kill, bracket orders, market-on-close, limit-on-close, immediate-or-cancel, etc.