I assume here that you know what SGML is but are not up to speed on what those four media are.
Sign language has nothing to do with captioning.
Closed-captioning Captions transmitted in the form of a code. Nearly all North American TVs carry decoder chips as standard equipment now. Open-captioning Captions that are an indelible part of the picture and are always visible. Captions denote sound effects meaningful to the narrative or to understanding the program. Captions move to indicate the position of the speaker.