The MassOrder(35=DJ) message is used to add, modify or delete multiple unrelated orders using a single message. Apart from clearing related attributes, only the key order attributes for high performance trading are available.
Additional detail from the official source
The behavior of individual orders within a MassOrder(35=DJ) message may vary depending upon its attributes, e.g. OrdType(40) and TimeInForce(59). Individual orders may be modified or deleted/canceled using OrderCancelReplaceRequest(35=G) and OrderCancelRequest(35=F) messages. Each of the orders in the MassOrder(35=DJ) message are to be treated as stand-alone individual orders.
Message structure 1330 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.