SRT: words and timing
SRT contains numbered caption segments, start and end times, and text. It is useful when a platform or editor accepts a separate subtitle file. Your custom font, colors and animation are not carried by SRT; the receiving player controls presentation.
ASS: styled subtitles
ASS can preserve styling and animation instructions for software that supports them. The appearance depends on the receiving renderer and available fonts. Test the file in the actual destination rather than assuming every player will reproduce the same look.
MP4: captions in the picture
Choose MP4 when you want the caption appearance permanently rendered into the video. Those captions are part of the image and cannot be turned off by the viewer. This is often convenient for video posts where a specific visual look matters.
Keep an editable master
A burned-in MP4 is a finished output, not a replacement for your editable project. Save the project and source media so you can correct text or make another format later. LocalCaptions keeps previous completed exports with unique filenames.
A useful delivery combination
When a client or platform supports both, a reviewed MP4 and a separate SRT can serve different needs. Confirm the recipient’s requirements and avoid displaying burned-in and player-generated captions at the same time if that duplicates text.