Attribute Numer: 18
Value: String

The Reply-Message attribute is used to send a message back to the client
in response of another packet.
RFC2865 describes it as easy as:

"This attribute indicates text which MAY be displayed to the user.
When used in an Access-Accept, it is the success message.
When used in an Access-Reject, it is the failure message. It MAY
indicate a dialog message to prompt the user before another
Access-Request attempt.
When used in an Access-Challenge, it MAY indicate a dialog message
to prompt the user for a response.
Multiple Reply-Message's MAY be included and if any are displayed,
they MUST be displayed in the same order as they appear in the packet.
A summary of the Reply-Message Attribute format is shown below. The
fields are transmitted from left to right."