OperationError

The field in the input that caused the operation error.

message with inline formatting — typically the same sentence plus a link to wherever the viewer can act on the error ("This division is now full. [Pick a different one](…)").

Null when there is nothing to add, which is the common case: a client that renders this should fall back to message, and message alone must always be a complete, useful sentence. Only ever an enrichment, never the sole carrier of meaning.