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You can't process the backlog on a trading platform. If i put in a trade at 2:20 pm and the system goes down, I don't want my trade to execute next morning at market open. That's insane. Especially the RH flavor of YOLO infinite leverage call option nonsense.

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Exactly, you have to default to fill or kill within the trading day. You just can’t treat certain products like a standard queue... sometimes time is the most important component

FYI, FillOrKill/ImmediateOrCancel are not the same as a day order.

FoK/IoC means “do not queue this order”. It’s immediately filled (or not) (or, for IoC, partially) based on whatever orders are already in the book, and then you’re done.

Whereas a day order is queued until the end of the day or until it’s filled, whichever comes first.

True for RH trades but I'm sure there is a lot of other data being handled - such as market data.

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