Variant of the specified [Speech
model][google.cloud.dialogflow.v2beta1.InputAudioConfig.model] to
use.
See the Cloud Speech
documentation <https://cloud.google.com/speech-to-text/docs/enhanced-models>__
for which models have different variants. For example, the
"phone_call" model has both a standard and an enhanced variant. When
you use an enhanced model, you will generally receive higher quality
results than for a standard model.