When Ediff is running, the mode line of ‘Ediff Control Panel’ buffer shows the current difference number and the total number of difference regions in the two files.
The mode line of the buffers being compared displays the type
of the buffer (‘A:’, ‘B:’, or ‘C:’) and (usually) the file name.
Ediff tries to be intelligent in choosing the mode line buffer
identification. In particular, it works well with the
uniquify.el and
mode-line.el packages
(which improve on the default way in which Emacs displays buffer
identification). If you don't like the way Ediff changes the mode
line, you can use ediff-prepare-buffer-hook to
modify the mode line.