What is a Forced Narrative Subtitle?
A Forced Narrative (FN) subtitle is a text overlay that clarifies communications or alternate languages meant to be understood by the viewer. They can also be used to clarify dialogue, texted graphics or location/person IDs that are not otherwise covered in the dubbed/localized audio. To enable the same viewing experience across multiple countries and devices, FN subtitles are localized and delivered as separate timed text files. The picture in your primary video (A/V MUX) that would otherwise have subtitles is required to be delivered as a non-subtitled file, or textless. Subtitles, both full and FN, are not burned-in over picture.
For a detailed explanation, please review the article Understanding Forced Narrative Subtitles.