In TWS help, the overfill protection is discribed as this: Dictates that only one order in the group will be routed to an exchange at a time. This completely removes the possibility of an overfill. This is what I am wondering: how does IB determine which order in the OCA group got sent to the exchange? For example, if one of the orders is an Island hidden limit order, and if IB doesn't send it to the exchange, that hidden order is practically useless. Does IB use some algorithm to determine which of the orders actually sent?