'Date Added' or 'Date Modified' for Smart Playlists?

I'm confused about Smart Playlists...

What is the difference between 'Date Added' and 'Date Modified' for Smart Playlists?

I have backed up all my iTunes songs on 10/31/05.
I changed something in one of those songs on 11/1/05. Gave it a different Star rating.
I made a Smart Playlist with 'Date Modified is after 10/31/05'.

The Smart Playlist ignores my modified Star Rating...
and also whatever change I do in Comments or other fields.

What does 'Date Modified' actually mean?!
I interpret Date Modified as the DATE where some/any field info of a song has been MODIFIED... and by making a Smart Playlist I expect to find these changes... is that a wrong interpretation?

How can I back up any changes in any info on a song AFTER the date of the former Back Up?

P.S. I've added 3 songs on 11/1/05. When I make a Smart Playlist with 'Date ADDED is after 10/31/05' or one with 'Date MODIFIED is after 10/31/05', the result is exactly the same : just those 3 songs...

Or do I do/see some/several things wrong?

Hope someone helps me out of this confusion...