Shared secret between the Remote Access server and the specified external RADIUS server which is required for successful ...

Shared secret between the Remote Access server and the specified external RADIUS server which is required for successful communication between the two servers. Note that the secret is specified in clear text. 
This parameter can be configured only if the Type parameter is specified to be ExternalRadius.
If there are no pre-existing RADIUS servers configured for authentication then it is mandatory to configure one by specifying at least a Radius server and shared secret. Default values can be used for the other parameters.