The exports (both live and manual) include a column “Additional details” that seems to be populated mostly just for fintech accounts (Monzo, Revolut, Starling, Wise), with just Nationwide as an exception.
The data in this field is not exposed in the mobile/web apps, and it doesn’t seem to be editable in any way (it doesn’t change if the custom name or notes are changed).
It seems to be the original data counterpart of the editable “Notes” field (just like “Merchant” and “Custom Name” are the user-editable versions of “Counterparty” which is preserved in the exports even if manually overridden in the app).
Any particular reason the data in this field isn’t available in the app? Defaulting the notes to the value of this (if set) looks pretty straightforward.