In the output of "aptitude update", what do "Ign" and the other codes mean?

When I run sudo aptitude update from the console, I see output with lines such as this:

Get:1 http://security.ubuntu.com lucid-security Release.gpg [198B]
Ign http://security.ubuntu.com/ubuntu/ lucid-security/main Translation-en_GB

or this:

Hit http://security.ubuntu.com lucid-security Release.gpg
Ign http://security.ubuntu.com/ubuntu/ lucid-security/main Translation-en_GB

scrolling by.

It's fairly clear that Get:N indicates a download, Hit indicates when I've got something already, and that whatever Ign lines are, they can appear even if source.list has no comments, so they're not 'skipping comment' indicators as I've seen some people say.

After a fair amount of googling, though, that's as far as I've been able to get, so I have two questions:

1. What exactly does the Ign code indicate?
2. Is there any documentation (official or otherwise) that lists all the available codes along with what they're for?

Solution 1:

What exactly does the Ign code indicate?

It means Ignored. That is, your local file is too recent to be refetched.

Is there any documentation (official or otherwise) that lists all the available codes along with what they're for?

I don't know. I wouldn't be surprised if there was none, perhaps because it seems obvious, or because of laziness.