Administration Console Online Help
Base64DecodingRequired—Specifies whether the request header value or cookie value must be Base64 Decoded before it is sent to the Identity Assertion provider. This box is checked by default for purposes of backward compatibility; however, most Identity Assertion providers will uncheck this box.
If false, then the server is not base64 decode the token before passing it to the Negotiate Identity Assertion provider. This attribute defaults to true for backwards compatibility but most providers probably want to set this to false.