When the Serialize option is set to OFF, transactions are processed in the order they are encountered, then they are placed in the first available buffer. Buffers are sent to parsing engine (PE) sessions and PEs process the data independently of other PEs. In other words, transactions might occur in any order.
If the Serialize option is not specified, the default is OFF unless the job contains an Upsert operation, which causes Serialize to switch the default to ON.