What's a unique identifier in listsinceblock output? I am trying to understand if there a unique identifier for transactions. I didn't find anything about that mentioned in listsinceblock's help.

{
  "involvesWatchonly": true,
  "account": "",
  "address": "mvd6qFeVkqH6MNAS2Y2cLifbdaX5XUkbZJ",
  "category": "receive",
  "amount": 10,
  "label": "",
  "confirmations": 69,
  "blockhash": "62967ce950fc03b14179e58f42e426b63cbfbf63c2b9e59acb6a719a09c23b94",
  "blockindex": 58,
  "blocktime": 1524872287278,
  "txid": "aac9b2d7bb7b3476a83a5ac0c2fe21625bcd961ed43a5c5fd3eacf6536dc81d5",
  "vout": 7,
  "walletconflicts": [],
  "time": 1524872276635,
  "timereceived": 1524872276635,
  "bip125-replaceable": "no"
},
Each transaction has a unique identifier which is the txid. It is the hash of the transaction and each transaction's hash is unique.