Constant EXIT_WRITE_BACK_FAILED 

pub const EXIT_WRITE_BACK_FAILED: u8 = 3;

A tune/simulate run completed the MRFT test itself, but writing the selected PID constants back to the DCS failed (the write was rejected, errored, or its confirmation readback didn’t match). Distinct from both EXIT_SUCCESS (nothing to report) and EXIT_FAILURE (the test itself never produced a result) so an unattended --write-pid/--yes run can tell “the test ran fine but the loop was NOT updated” apart from either of those. See commands::tune::TuneOutcome and AGENTS.md’s cli-automation section.