gnus: Symbolic Prefixes
9.3 Symbolic Prefixes
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Quite a lot of Emacs commands react to the (numeric) prefix. For
instance, ‘C-u 4 C-f’ moves point four characters forward, and ‘C-u 9 0
0 I s s p’ adds a permanent ‘Subject’ substring score rule of 900 to the
current article.
This is all nice and well, but what if you want to give a command
some additional information? Well, what most commands do is interpret
the “raw” prefix in some special way. ‘C-u 0 C-x C-s’ means that one
doesn’t want a backup file to be created when saving the current buffer,
for instance. But what if you want to save without making a backup
file, and you want Emacs to flash lights and play a nice tune at the
same time? You can’t, and you’re probably perfectly happy that way.
I’m not, so I’ve added a second prefix—the “symbolic prefix”. The
prefix key is ‘M-i’ (‘gnus-symbolic-argument’), and the next character
typed in is the value. You can stack as many ‘M-i’ prefixes as you
want. ‘M-i a C-M-u’ means “feed the ‘C-M-u’ command the symbolic prefix
‘a’”. ‘M-i a M-i b C-M-u’ means “feed the ‘C-M-u’ command the symbolic
prefixes ‘a’ and ‘b’”. You get the drift.
Typing in symbolic prefixes to commands that don’t accept them
doesn’t hurt, but it doesn’t do any good either. Currently not many
Gnus functions make use of the symbolic prefix.
If you’re interested in how Gnus implements this, 
Extended
Interactive.
Info Catalog
gnus: Interactive
gnus: Various
gnus: Formatting Variables