The ioctl(2) intended to provide more details about the cause of the down for the link.
Eventually we might define a comprehensive list of codes for the situations. But interface also allows the driver to provide free-form null-terminated ASCII string to provide arbitrary non-formalized information. Sample implementation exists for mlx5(4), where the string is fetched from firmware controlling the port.
Currently, the interface is only utilized by ifconfig(8) to provide the hint to user, if available.