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. Before built-in decoders were required by law, you needed an external decoder, which are still available in declining supply. 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.