CAPABILITY_NOT_IMPLEMENTED
HTTP 501No
This capability has no live implementation/adapter yet — a DEVELOPMENT gap, not config. Adding a vendor key will NOT make it work (distinct from VENDOR_DOWN = implemented but no usable key).
By default a failed call returns a result with an error instead of throwing, so your business flow is never interrupted.
Published error codes are permanent: their meaning never silently changes, and deprecated codes keep returning for at least 12 months.