[Exim] Seen vs. Unseen in string expansion

In the course of writing a system-wide filter, I have seen several
references in the Exim documentation to the commands "seen" and "unseen" as
relates to string expansion. I have spent a fair bit of time reviewing the
Exim WWW site and related documentation, but cannot find a clear, simple
explanation of these commands and what they indicate in regards to string
expansion.

Any simple explanation of these terms would be VERY appreciated.

Thank you.

Drew Berendts
Internet Security Engineer
Verio, Inc.