procmail splitting, multiple mail sources, and stuff

[nobrowser]

I use Gnus 5.9.0.

Unfortunately, the section of the manual that explained relationship
between procmail splitting and Gnus groups ("Mail & Procmail") seems
to be gone, even though FAQ 4.1 still refers to it.  Anyway, my
problem/question is: is it possible to parametrize the nnmail select
method to only scan a particular mail source?

Let's say I read most mail locally, and I split it with procmail
into multiple spool files in a directory.  I make a mail source
for that directory, and I know that mail groups named after the spool
files will magically fetch articles only from the corresponding file.
So far so good.

But now assume I have a separate POP account, and I want a separate
group for mail that arrives in that account.  I don't want the new
group to get mail from the spool files, or vice versa.  How can I do
this?

It seems to me each mail source should have a "name" that could then
be used as the "server" parameter for nnml methods.  Perhaps
something like this already exists and I just lost my way in
the manual?