Blu-ray is a disc media that is capable of storing HD video, essentially superseding the DVD. Blu-ray does not support closed captions; it does, however, support subtitles. Subtitles for the deaf and hard-of-hearing (SDH) are subtitle files in the source language of the video that also include important non-dialogue audio sound effects and speaker identification. This makes closed captions and SDH identical in many ways: the main difference is that they are encoded differently, and most HD disc media (including Blu-ray) cannot hold closed caption files. Note that any disc playing through an HDMI connection will not support closed captions, either. For these reasons, SDH are included on Blu-ray and other HD disc media (like HD DVDs) to make video content accessible. A common way to add SDH to Blu-ray is by creating the file format BDN.xml, along with .png (image) files.
While accessibility for the deaf and hard-of-hearing is an important benefit of SDH for Blu-ray, SDH make video content accessible in a number of other ways, as well. The BBC published a study by Ofcom that showed that 80% of people who use captions have no hearing impairment at all; it could easily be implied that the same thing is true of SDH. SDH ensure that none of the dialogue will be missed: they make video accessible in noisy areas; they make video comprehensible to viewers whose first language is not English; they help people understand thick accents or muddled voices; and they help people decipher difficult concepts.