Needing clarity about agentized group - agentized server

[Harry Putnam]

Hopefully this longish post will not be TLTR as I've seen happen from
time to time.
I am sorry for the windyness, but having a bit of trouble describing
what I want to know ... more briefly.

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Recently started using gnus-agent again after a very long hiatus.

I'm getting confused about the repercussions of or differences between
a group whos server is agentized, but when the group itself is not
agentized.

It seems that gnus behaves as if groups are agentized, that is, the
face on new message appears completely different than it did
before.. but the same as in agentized groups.  Further; any messages I
read are kept .. as if the group were agentized.

But the real rub comes if I tell the agent do download un-read posts.

That starts a massive pull on reading even when there's only maybe
only dozens or at most a couple hundred new messages in agentized
groups.

gnus appears to be downloading from groups that are not agentized.
But that groups server is agentized.

For example, I have 38 agentized groups across 3 servers.  But when I
check all the directories under News/agent I find 44 groups with some
number of messages.

I'm loath to run a serious test, like for example pressing `J s' in
group buffer (‘gnus-agent-fetch-session’) and just `let 'er rip' for
fear that gnus will try to download from all groups under the
agentized server whether or not such groups are actually themselves
agentized.

If the new setup of gmane is similar to the old one under `larsi' then
it is possible to become blackballed from gmane ... and if that
happened I'm not sure there would be any timely way to resolve that.

Years ago that happened to me more than once, but was resolvable by
direct appeal on `gmane.discuss'.  And fairly timely too.

There is also `J u' to use but I'm not totally clear on the
difference.. I mean other than that J u applies to current group
only.

I guess one might be able to process mark all the agentized groups and
use `J u' but that also seems to invoke a hefty read of many many lines
of something.

Again loath to let it play out.

Here is a fear I have:  Currenly I've slowly downloaded many thousands
of posts.  But, I've done it only to a point ... for example in a
Foreign server (Used to hopefully avoid any excessive use of gmane):
   nntp+enews.newsguy.com/gnu/emacs/help      59900

Nearly 60,000... but there are many many thousands more even older
... Depending on what;
`Fetch all eligible articles in the current group' means exactly.
That is, if non-downloaded posts is considered `eligible' then gnus
might try to download the reported 300,000+ still on that group that
I have not downloaded (and do not wish to download).

The doc string on gnus-agent-fetch-groups (`J u' in group buffer)
Appears to be pretty clear

  `Put all new articles in the current groups into the Agent'

I doubt `new' might describe the other thousands of undownloaded posts
on the example group.  But, I'd like to know for sure.

The doc string on `gnus-agent-fetch-session' (`J s' in group buffer)is
a good bit more ambiguous
  `Fetch all articles and headers that are eligible for fetching'

Here, `eligible' might easily mean those other thousands of posts
available in the example group.

For those reasons I'd like some clarity

What `J s' means in practice?
What `J u' means in practice?

What difference is there in a group that is not agentized but its
server is,
compared to a group where both server and group are  agentized?

To get the desired result in the case I described should I be just
agentizing groups but not servers?

How would that effect `J s' and `J u'?