There are several document formats defined in the ISO 20022 standard for exchanging transaction history between banks and 3rd party customer software; the ones I’m interested in are camt.052 for realtime account information and camt.053 for historical data. I’m struggling with how I’m supposed to uniquely identify transactions in these documents, so that I don’t process the same transaction twice on repeated requests to the bank’s API. There are several fields which look promising, most notably AcctSvcrRef (short for Account Servicer Reference, if I’m not mistaken), but I’ve found when trying to interface with two separate global-tier banks that these fields are often either missing entirely, or filled with a placeholder blank value.