High Efficiency video mode (HEVC) is brand new, developed by Apple in 2015 or 2016. It may be necessary to wait for High Sierra and/or next iMovie udate before all of the compatibility kinks are worked out. It uses H.265 codec whereas H.264 codec and JPEG is what has been used to date in iMovie and most other devices, including Windows.
iOS 11 sets the default at High Efficiency, but you can switch between the High Efficiency format and the Most Compatible format under iOS Settings > Camera> Formats. If you don't change the default you will be recording all of your vids in HEVC and your photos in HEIF (rather than JPEG). I understand that HEIF is compatible with Photos 2.0 under Sierra, but don't know that from experience.
HEVC is advertised at being able to reduce file size by about 50%, and be better for managing very high res videos like 4K. The downside is incompatibility with non-Apple devices, at least for the present. I am not sure whether HEVC and HEIF are compatible with older versions of Mac OS. I think we will need to wait and see, pending any further info from Apple.
-- Rich
