apt-move question

[Paul E Condon <pecondon@quiknet.com>]

I am attempting to set up a partial local mirror of debian/ using
apt-move. There is nothing special about my Debian
installation that I am aware of. (I am much too new to Debian to do
anything special deliberately!)
I have configured apt-move to use ftp.us.debian.org for non-non-US.

When I run "apt-move -t update", I get the following messages:
# apt-move -t update

Updating Packages and override files...
Getting: distribution names
Getting: stable main Packages.gz
Getting: stable main override.gz

Creating Lists...
Error: makelist: No master override files exist!

questions:
1st: What are override files (master and/or otherwise)?
2nd: Where do I find a discussion of apt that covers override files?

Thanks,
Paul