What is this icon on the upper right conner means?
I couldn’t find about it in the manual.

The image appears when you lock it in horizontal mode from the menu. When you lock it in vertical mode, the image flips as well. You can do this in the swipe-down menu.

Just an indicator that the video is in landscape mode; it's not a selectable command icon.

Hi there. Thank you for reaching out. This icon means that you are currently using Horizontal mode. As mentioned by marc above, the icon will change if you switch the shooting mode to vertical shooting. Should you have other inquiries, feel free to contact us. Thank you for your valued support.

Thanks for asking this question. I had looked everywhere and couldn't find anything and was going to post about it and then found your thread.  

Marc_v_L Posted at 10-31 01:13
The image appears when you lock it in horizontal mode from the menu. When you lock it in vertical mode, the image flips as well. You can do this in the swipe-down menu.

Thanks for solving that mystery for me also.