GNU Emacs Manual
A summary is a buffer containing one line per message to give
you an overview of the mail in an Rmail file. Each line shows the
message number and date, the sender, the line count, the labels, and
the subject. Moving point in the summary buffer selects messages as
you move to their summary lines. Almost all Rmail commands are valid
in the summary buffer also; when used there, they apply to the message
described by the current line of the summary.
A summary buffer applies to a single Rmail file only; if you are
editing multiple Rmail files, each one can have its own summary buffer.
The summary buffer name is made by appending `-summary' to the
Rmail buffer's name. Normally only one summary buffer is displayed at a