[exim] unseen delivery

I just "learned" something about unseen deliveries which
I can't believe skipped by me all these years.

In a user forward file I would have:

# Exim filter

deliver alternate@???
unseen
finish

...............

and this was treated as SEEN!
So I dig in the book, and clearly find that "unseen" must
precede the deliver command. My bad.

But why would exim even "permit" (without an error message)
a lone "unseen" line in the filter if it weren't acting on it?
I have always considered that the keyword "unseen" would set
an unseen flag. This does not appear to be the case.

Is the keyword "seen" also not going to be triggering a
significant delivery if it appears on a line all by itself?

Thanks, Jim