Mobile Letterbox is similar to Square Letterbox. 9:16 aspect ratio with letterbox padding on the top and bottom.
This can be great for Instagram stories wherein you can incorporate a video /image slideshow along with the text keeping in mind your brand color.
To create a video with Mobile Letterbox and Letterbox:
Step 1: Click the Project Settings button.
Step 2: Click Resize and choose any of the letterbox formats.