The past couple years, Penn's ranking system has been the following:
Accepted - Congrats!
Top of Tier 1 Waitlist - You're going to be the first off of the waitlist when it moves. A LOI would solidify things at this point. Don't expect a scholarship though. If you have this status it WILL say so on the website and on your letter.
Tier 1 Waitlist - This is the real waitlist. Good luck.
Tier 2 Waitlist - Penn's form of a rejection. It's rare that they dig down this far.
The waitlist usually does not move until after Penn Preview. Sometimes they do invite top of tier 1 waitlistees to Penn Preview if they express an interest in coming. The waitlist is ranked, but the ranks change depending on things like LOI, class makeup, etc... They will NOT admit to having rankings and will NOT tell you where you are ranked.
Now for the MD/PhD folks: The competitiveness here swings wildly from year to year. In my year, they took some people from Tier 2 waitlist, and last year they didn't even have room for Top of Tier 1 people because the waitlist hardly moved at all. Some of the people invited back for Preview didn't even get in, which was kind of cruel. If they don't mention paying for your travel expenses to Penn Preview, do mention that you'd like them paid for. They will, but sometimes only if you ask. That does not apply for MD students, except I hear rumors about certain VIPs (disadvantaged, URM, scholarship, etc).
I hope that clears it up for you guys. Good luck!