#include <Charset.h>
This is NOT a widely used concept any longer. Its largely been replaced by UNICODE. But - it IS used in HTTP, and MIME, so its pretty important to continue supporting. https://www.w3.org/International/articles/http-charset/index#charset
Information tagged with a Charset is used to select a CodeCvt object, which maps (typically from) that character set to UNICODE.
See https://www.iana.org/assignments/character-sets/character-sets.xhtml