The traditional, line-21 style captions are formally documented and defined in the EIA-608 standard (although as I recall, it wasn't 100% complete - to truly define a CC decoder function I needed a couple of additional documents, the old Telecaption-II report and perhaps one of the FCC documents).
The newer style captioning used in HDTV broadcasting is defined in EIA-708. It is a more powerful method of captioning. The problem of international fonts was addressed, and proportional fonts are also allowed. The user has some control over the display of captioning. For example, font size can be adjusted and positioning can be overridden by the user. The mechanism for carrying these new captions also reserves bandwidth to simultaneously carry the older line-21 style captions as well.
As I understand the current legal situation in the USA (please correct me if I'm wrong...), broadcasters are required to transmit either just EIA-608 captions, or both 608 and 708 captions (in other words, I have heard that sending just EIA-708 is not allowed at present).
Thanks for the comments so far - keep them coming please.
