The docs for gnus-summary-line-format mention that "%U" produces "R",
"K", "-", or " ".  However, they fail to mention "r", for example.  The
first clue I could find about that mark was in "9.18.4 Spam and Ham
Processors" of all places, where it drops the hint that "r" means
"explicit delete" (!).  Users *really* need a little table that gives a
complete guide to the built-in marks, so that when they look at the
summary buffer, they know what it means.

Same goes for the other Gnus buffers that support marking, BTW.  K and U
in the server browsing buffer are confusing as well.

Package: gnus
Version: 5.110018
No Gnus v0.18
GNU Emacs 23.3.1 (x86_64-apple-darwin, NS apple-appkit-1038.35)
 of 2011-03-09 on black.porkrind.org
200 news.gmane.org InterNetNews NNRP server INN 2.5.1 ready (posting ok)
101 Capability list:
VERSION 2
IMPLEMENTATION INN 2.5.1
AUTHINFO USER
HDR
LIST ACTIVE ACTIVE.TIMES DISTRIB.PATS HEADERS NEWSGROUPS OVERVIEW.FMT
OVER
POST
READER
STARTTLS
.