emacs file encoding choices: with-signature?

[Xah Lee]

when using set-buffer-file-coding-system
(emacs 23), i get the following choices with utf-8:

Possible completions are:
utf-8-auto      utf-8-auto-dos
utf-8-auto-mac  utf-8-auto-unix
utf-8-dos       utf-8-emacs
utf-8-emacs-dos         utf-8-emacs-mac
utf-8-emacs-unix        utf-8-mac
utf-8-unix      utf-8-with-signature
utf-8-with-signature-dos        utf-8-with-signature-mac
utf-8-with-signature-unix

what is the "with-signature" mean? is it the unicode BOM marker?
and what about "emacs"? Is that the emacs 22's internal encoding
system?

  Xah
∑ http://xahlee.org/

☄