Some response fields are enum-typed. In the future, individual services may return values not covered by existing enums for that field. To address this possibility, every enum-type response field has an additional value named "UNKNOWN_ENUM_VALUE". If a service returns a value that is not recognized by your version of the SDK, then the response field will be set to this value. Please ensure that your code handles the "UNKNOWN_ENUM_VALUE" case if you have conditional logic based on an enum-typed field.