Some schools put most people who interview on a waitlist, and then later formulate an alternate list of the people at the top of the waitlist. The people on the alternate list are offered a spot in the class as they become available. For some schools, an alternate list may be the same thing as a waitlist... since you were put on an "alternate list" withougt being on a waitlist first, it probably is the same thing at SLU.
SLUs alternate list is the same as a waiting list. and be prepared to wait. i interviewed last year in jan, got put in the alternate list in feb and waited the whole spring/summer without a peep until i got the rejection in july. this was only my case, so you may hear news earlier, but just be prepared for a long wait.