You can change the aspect ratio of your video from this option. If you do not want to change the aspect ratio and to keep the existing ratio then select Standard option. If you choose any other option, a black bar will be added to either side of the video depending on your selection. 


Standard - Keep your video as it is. 


Vertical - Optimise your video for mobile viewing. Use for sites like Instagram and other mobile apps. 


Top and Bottom - Adds both bottom and top text to the video that sticks during the duration of the video. 


Top - Adds top text to the video that sticks during the duration of the video. 


Bottom - Adds bottom text to the video that sticks during the duration of the video.