The ID for a transaction. This is used for retrieving receipts and records for a transaction, for appending to a file right after creating it, for instantiating a smart contract with bytecode in a file just created, and internally by the network for detecting when duplicate transactions are submitted. A user might get a transaction processed faster by submitting it to N nodes, each with a different node account, but all with the same TransactionID. Then, the transaction will take effect when the first of all those nodes submits the transaction and it reaches consensus. The other transactions will not take effect. So this could make the transaction take effect faster, if any given node might be slow. However, the full transaction fee is charged for each transaction, so the total fee is N times as much if the transaction is sent to N nodes.