8 codes that cover most of what an originator sees. Each one carries the decision: retry or stop, how long the receiving institution has to send it, what the guidance rests on, and when it was last checked.
On a rail fact, every detail line names the section or instrument it rests on and whether that is a rule, a law, operator guidance, or observed practice.
Not yet covered: Pay.UK does not publish the return reason list. Faster Payments returns are governed by the FPS Rules and the FPS Procedures, which Pay.UK's 2025 PFMI self-assessment, key consideration 23.1, says are available for review by participants' legal departments; 6 of the 14 return reasons a participant's developer documentation lists are not drafted here. Either only one public source family carries them, or the two families that carry them disagree on what they mean. The held reasons are listed by number in docs/rails/uk-fps.md; which of the one to three working days applies to a given return: Pay.UK states the range publicly and says it depends on the answer the receiving participant gave to the original payment, and the mapping is in the FPS Procedures; whether any of these reasons may be raised by a party other than the receiving institution. No public source read says, and the code that covers a return at the original sender's request plainly starts elsewhere.