What exactly is chinese-big5?

[Eli Zaretskii <eliz@gnu.org>]

Emacs 22.2 supports the chinese-big5 encoding.  However, I cannot find
anywhere the precise description of which flavor(s) of BIG5 is/are
supported.  The Wiki page http://en.wikipedia.org/wiki/Big5 describes
half a dozen of extensions to the original Big5 encoding, so it would
be good to know which one(s) we support.

The specific situation where I needed to know this was when I was
handed a file with what was supposed to be Chinese text and was asked
to convert it to UTF-8.  detect-coding-region suggested chinese-big5
as the only Chinese encoding for the non-ASCII characters in the file,
so I tried that.  Interestingly enough, both `recode' and `iconv'
refused to convert the file, no matter what flavor of Big5 (including
cp950) I tried, but Emacs read the file with no problems and produced
what seems like a valid UTF-8 encoding.  `iconv' 1.12 supports quite a
few Big5 flavors, but they all choked on some characters in the file.

So what exactly is chinese-big5 in Emacs, and how come it succeeds
where the latest `iconv' fails?  In particular, should I worry about
possibly incorrect conversion by Emacs, where `iconv' barfs (the file
is very large and I cannot proofread all the converted strings)?

TIA