Here’s a good explanation on  Netflix’s website on Forced Narrative: https://partnerhelp.netflixstudios.com/hc/en-us/articles/217558918 A Forced Narrative 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.   If you are creating closed captioning or SDH for Netflix, Forced Narrative subtitles are only displayed if full Subtitles and CC are set to “off” in the user’s playback settings. When the user activates a full Subtitle or CC file, the FN subtitle does not display and for this reason, we require that all Forced Narrative events are also included in each full Subtitle and SDH/CC file.