What's the prefix of message-send used for?

[XeCycle]

Hello, I want to write a wrapper to call `message-send-and-exit', and
I'm confused how should I deal with that &optional arg.

Indeed I wanted a better way of signing my messages, and came up with
this idea: C-c C-c to send a signed message, and C-u C-c C-c to send an
unsigned message.  However I don't know if this conflicts with any Gnus
feature, and the &optional arg is not fully documented.  Is it used for
something?  If so how should I pass the arg to it?

-- 
Carl Lei (XeCycle)
Department of Physics, Shanghai Jiao Tong University
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