Value: 4
Indicates the type field contains an external type name.
Used to encode custom payloads. When creating new records use the helper NdefRecord.CreateExternal(string, System.String, System.String).
The external-type RTD format is specified in NFCForum-TS-RTD_1.0.
Note this TNF should not be used with RTD_TEXT or RTD_URI constants. Those are well known RTD constants, not external RTD constants.