Confused about g-d-adaptive-score-alist manual description

[John Griffith <griffith@sfs.nphil.uni-tuebingen.de>]

The manual gives the following example:

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     (defvar gnus-default-adaptive-score-alist
       '((gnus-unread-mark)
         (gnus-ticked-mark (from 4))
         (gnus-dormant-mark (from 5))
         (gnus-del-mark (from -4) (subject -1))
         (gnus-read-mark (from 4) (subject 2))
         (gnus-expirable-mark (from -1) (subject -1))
         (gnus-killed-mark (from -1) (subject -3))
         (gnus-kill-file-mark)
         (gnus-catchup-mark (from -1) (subject -1))))

As you see, each element in this alist has a mark as a key (either a
variable name or a "real" mark--a character).  Following this key is a
random number of header/score pairs.
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What does a element of this list do if has NO header/score pairs?  For
instance, what does `(gnus-unread-mark)' do?  This does not seem to be
addressed.

Also, other questions include:

  1) If more than one element applies to an article, are all
     header/score pairs used? (I assume they are).  For instance, if
     an article is both read and marked dormant will the author get
     raised 4+5 = 9 points?

  2) If several messages have the same subject (or the same author)
     will the scoring rules be applied multiple times?  For instance,
     if there are 26 articles in some thread which I didn't read but
     catchup'd, would that subject be decreased by -1 or -26 points?

  3) A list of all (reasonable) marks would be useful too.