Audio optimization and video optimization (default) are two different approaches to syncing dubbing for a video in another language. Here's a breakdown of what each term means:

You can find these options in advanced settings.
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​Video Optimization: This process focuses on preserving the original video length while adjusting the audio to achieve the best results. The goal is to avoid any visual artifacts in the video caused by altering its duration. However, in video optimization, the audio is not prioritized, which means that adjustments may be made to the video to maintain the original duration.
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Video optimization is typically preferred when maintaining the original video length is crucial or when there are significant motion or human elements within the video frame that need to be preserved.

Audio Optimization: In contrast to video optimization, audio optimization prioritizes the quality and naturalness of the audio while allowing for changes in the video length. The primary focus is on delivering excellent verbal quality in dubbed audio. This approach may involve altering the duration of the video to synchronize it with the audio track effectively.
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While the audio quality is enhanced, the video may experience jitters or slight disruptions due to these modifications.
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Audio optimization is typically preferred when working with animated videos, presentations, or content intended for knowledge transfer where there are no human subjects within the video.
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In summary, video optimization aims to maintain the original video length while adjusting the audio, whereas audio optimization prioritizes the audio quality and may involve modifying the video length to synchronize it effectively. The choice between these approaches depends on the specific requirements of the video and the desired outcome.