Short answers, pulled from the story.
In British English, subtitles usually refer to those designed for deaf or hard-of-hearing audiences while SDH distinguishes between types. Subtitles for the deaf or hard-of-hearing refers to regular subtitles in original language where important non-dialogue information gets added alongside speaker identification.
Film importers in Germany Italy France Switzerland Luxembourg Austria San Marino Liechtenstein Monaco Slovakia Hungary Belarus Andorra Spain Canada New Zealand Ireland United Kingdom decided to dub foreign voices upon arrival of sound film in late 1920s early 1930s. Rest of Europe elected display dialogue as translated subtitles instead creating distinct regional preferences persisting today.
The only significant difference between SDH subtitles and closed captions lies in their visual appearance. SDH subtitles display with proportional fonts used for translation subtitles whereas closed captions appear as white text on black bands blocking large portions of view.
Christopher Nolan stated filmmakers complained about inaudible dialogue though he claimed it was intentional mixing choices. Studios show increasing unwillingness to spend money re-recording unclear dialogue leading to higher subtitle usage rates.
A 2021 UK survey found that eighty percent of viewers between ages eighteen and twenty-five regularly used subtitles while less than a quarter of those aged fifty-six to seventy-five did. Advances in microphone technology enable actors to speak more softly creating incomprehensible dialogue requiring subtitle assistance.