2003 December 2, “Anime Network - Missed it by that much...”, in rec.arts.anime.misc[1] (Usenet):
the International Channel used hard-subs though, but most video=DVD masters nowadays are soft-subbed
2018, Taylore Woodhouse, " A Community Unlike Any Other": Incorporating Fansubbers into Corporate Capitalism on Viki.com[2], University of Texas at Austin:
Independent K-drama fansubbers felt that soft-subbing afforded them a bit of legal protection since they usually did not distribute pirated video files themselves.
2020, KokSheik Wong, ChuanSheng Chan, Aprilpyone Maungmaung, “Lightweight authentication for MP4 format container using subtitle track”, in IEICE TRANSACTIONS on Information and Systems[3], volume 103, number 1, The Institute of Electronics, Information and Communication Engineers, →DOI, →ISSN, pages 2–10:
In other words, the MP4 file in question should be soft-subbed, i.e., a type of video that contains subtitles where its display can be turned on or off in the media player
