The difference is how captions are displayed
Open captions are always visible in the picture. Closed captions are a separate track that a compatible player can enable or disable. These terms describe delivery, not whether the words were made by AI or a person.
Choose open captions for a fixed visual result
A rendered MP4 keeps your caption size, position and design wherever it is reposted. It is useful for a social clip, but a viewer cannot remove the words or resize them independently. Save the original video for later changes.
Choose a closed track for viewer control
SRT or VTT can be used by a compatible platform as a subtitle track. The player controls appearance and access. Confirm that your upload destination accepts the format and language before relying on it.
One project can serve both
Review one subtitle timeline, then export a captioned MP4 or a separate file as needed. Test the result in the final player; a file that parses correctly still needs human review for wording and timing.