ACH return code
The short code, an R and two digits, that explains why a US ACH payment bounced.
When the receiving bank can't or won't post an ACH entry, it sends it back with a code from Nacha's list. The usual suspects: R01 insufficient funds, R02 account closed, R03 no such account, and R10 the customer says the debit was never authorized. Most returns have to happen within two banking days, but a consumer disputing an unauthorized debit gets 60.
That long tail is why American fintech apps so often hold deposits for days: money that already shows in the account can still come back as an R10 weeks later. Nacha also watches return rates per originator. Cross 0.5 percent unauthorized returns and you get a letter, and eventually you can lose access to the network altogether.