SDR (Standard Dynamic Range) is the traditional way of displaying video with a limited range of brightness and contrast. 709 (or Rec. 709) is a color standard used for video to make sure colors look the same on different screens. It sets the rules for brightness, contrast, and color balance so everything looks right when you watch the video on your device.
For more information regarding color processing in Mavis Camera, check out this dedicated support article: HDR and Color Space