Visio database question

I looked at all the forums and thought this one might be the best one to post this message.

I am trying to set up a Visio diagram of our database. I have looked at the examples and can understand some of it but have no idea what other things are and, as usual with a MS product, don't know where to look for the answers.

I can't reverse engineer this because it's not done in a database that Visio recognizes.

A quick question is: Some fields are primary keys and some are foriegn keys, but what are U1s and I1s (or 2s, etc.)?

Doug