The OrderCancelReject(35=9) message is issued by the recipient upon receipt of a OrderCancelRequest(35=F) or OrderCancelReplaceRequest(35=G) message which cannot be honored. Requests to change price or decrease quantity are executed only when an outstanding quantity exists. Filled orders cannot be changed (i.e. quantity reduced or price change, however, the recipient may support increasing the order quantity on a currently filled order).
Additional detail from the official source
When rejecting a OrderCancelReplaceRequest(35=G) (or OrderCancelRequest(35=F)) message, the OrderCancelReject(35=9) message should provide the ClOrdID(11) which was specified on the OrderCancelReplaceRequest(35=G) or OrderCancelRequest(35=F) message for identification, and the OrigClOrdID(41) should be that of the order being replaced or cancelled as specified in the OrderCancelReplaceRequest(35=G) or OrderCancelRequest(35=F) -- except in the case of CxlRejReason(102)=1 (Unknown Order).
When rejecting an OrderMassCancelRequest(35=q) message, ClOrdID(11) should be set to the ClOrdID(11) value of the OrderMassCancelRequest(35=q) message. OrigClOrdID(41) is not specified for a rejected OrderMassCancelRequest(35=q) message.
The ExecutionReport(35=8) message is the response to accept OrderCancelReplaceRequest(35=G) (or OrderCancelRequest(35=F)) messages.
Message structure 71 distinct fields
Source order, including nested components and groups. Group counts precede the repeated entries. “Required here” is relative to the enclosing component or group, not unconditional in the message. A dash means not marked required; conditional requirements remain in the source notes. This is a reference, not a counterparty-specific validator.
Includes nested components and groups. Escape clears the filter.
Trading partner LocationID (i.e. geographic location and/or desk) used when delivering messages via a third party. (Can be embedded within encrypted data section.)
Trading partner LocationID (i.e. geographic location and/or desk) used when delivering messages via a third party. (Can be embedded within encrypted data section.)
Always required for retransmitted messages, whether prompted by the sending system or as the result of a resend request. (Can be embedded within encrypted data section.)
Required for message resent as a result of a ResendRequest. If data is not available set to same value as SendingTime (Can be embedded within encrypted data section.)
The last MsgSeqNum value received by the FIX engine and processed by downstream application, such as trading system or order routing system. Can be specified on every message sent. Useful for detecting a backlog with a counterparty.
HopGrpRepeating groupCount tag 627—
Number of repeating groups of historical "hop" information. Only applicable if OnBehalfOfCompID is used, however, its use is optional. Note that some market regulations or counterparties may require tracking of message hops.
Unique order id assigned by institution or by the intermediary with closest association with the investor. to the cancel request or to the replacement order.
ClOrdID(11) which could not be canceled/replaced. ClOrdID of the previous accepted order (NOT the initial order of the day) when canceling or replacing an order.
Required when referring to orders that were electronically submitted over FIX or otherwise assigned a ClOrdID.