Identifier that can be used to correlate messages, transactions, log entries etc. The identifier should be unique across all relevant systems. It is the responsibility of the creator of the message/transaction/log entry/... to guarantee uniqueness. The
TransactionData can for instance be a GUID, or something based on site specific rules.
TransactionData is primarily used to simplify distributed debugging/tracing. To be able to easily search for the same thing across systems is very helpful.