Single-character ADDACS scheme reason code published by Bacs. Accepted values
(case-insensitive, whitespace-trimmed): "0", "1", "2", "3", "5", "6", "7",
"B", "C", "D", "E", "F", "G", "H", "I", "K", "L", "M", "N", "O", "P", "Q".

Surfaces on the resulting dd_mandate.suspended event payload (or, for mandates
still in PENDING_ACTIVATION, on dd_mandate.failed as the AUDDIS rejection
reason).

Example: "B" (Account closed).