You turn it on or off in Camera --> Camera settings (the icon that looks like a camera with a gear on it), then at the bottom of the little box that open, click on More. Video Caption is the third line down.
I never use dji app for editing. But my question remains unanswered.
Can you edit .srt file along with the video file so that the data displayed corresponds "exactly" to the edited movie file? The thing is that every video and photo can be improved with removing ie. quick Lt. Rt. movements, improper camera movements, etc. Editing is mandatory for a good movie.