For more information about HEVC, see this Apple Support article. If you’re using macOS 10.13 or later and Keynote 8.0 or later and you select this format, you can later add HEVC files to the reduced-size presentation or copy. Resize changes the final image dimensions (pixel count) by the given percentage. Movies in HEVC format have the best quality at the smallest file size, but they can be played only on devices with iOS 11, iPadOS 13, or later, and Mac computers with macOS 10.13 or later. Quality/Compression permits reduction of the exported file size. High efficiency (HEVC): This format option appears only if you’re using macOS 10.13 or later and your Mac supports hardware HEVC encoding.
Most compatible (H.264): Movies in your presentation are converted to H.264 format, which plays on most iOS devices, iPadOS devices, and Mac computers. Remove trimmed parts of movies and audio: If you trimmed the start or end of movies or audio clips (using the controls in the Format sidebar), the trimmed portions are deleted. Note: Whether or not you select this checkbox, if an image is in a format that can’t be viewed on iPhone or iPad, it’s automatically converted to a format that can.
