gnus-split-methods

gnus-split-methods is a variable defined in `gnus-art.el'.
Its value is

((gnus-article-archive-name)
 (gnus-article-nndoc-name))

Documentation:
*Variable used to suggest where articles are to be saved.
For instance, if you would like to save articles related to Gnus in
the file "gnus-stuff", and articles related to VM in "vm-stuff",
you could set this variable to something like:

'(("^Subject:.*gnus\|^Newsgroups:.*gnus" "gnus-stuff")
("^Subject:.*vm\|^Xref:.*vm" "vm-stuff"))

This variable is an alist where the key is the match and the
value is a list of possible files to save in if the match is
non-nil.

If the match is a string, it is used as a regexp match on the
article. If the match is a symbol, that symbol will be funcalled
from the buffer of the article to be saved with the newsgroup as the
parameter. If it is a list, it will be evalled in the same buffer.

If this form or function returns a string, this string will be used as a
possible file name; and if it returns a non-nil list, that list will be
used as possible file names.

You can customize this variable.