public com.google.protobuf.ByteString getUsageBytes() {
      java.lang.Object ref = usage_;
      if (ref instanceof String) {
        com.google.protobuf.ByteString b =
            com.google.protobuf.ByteString.copyFromUtf8((java.lang.String) ref);
        usage_ = b;
        return b;
      } else {
        return (com.google.protobuf.ByteString) ref;
      }
    }
<pre>
Output only. Usage of the sentence pair. Options are TRAIN|VALIDATION|TEST.
</pre>

<code>string usage = 4 [(.google.api.field_behavior) = OUTPUT_ONLY];</code>

@return The bytes for usage.
java-translate/proto-google-cloud-translate-v3/src/main/java/com/google/cloud/translate/v3/Example.java