Jennifer - I'm not certain, but I believe zone 1 is business class and Elite. One source says "priority" is also zone 1 but then I thought I read "priority" boards after business. Either way, priority should boarding before they start boarding the back rows.
Further research has me nearly convinced that Airtran uses a boarding procedure that is a combination of the rotating zone method (described on seat guru) and boarding window and middle seats before isle seats. So, after zone 1 (and priority boarding), people in the back few rows with window seats would go next and then window seats in another section. People near the middle with isle seats might be the last to board. I think seats in rows 12-16 are usually tickets sold with "priority boarding." And then factor in that I think they try to board people under one booking together. (So a family of 3, with 3 seats in a row wouldn't be split up during the boarding process, but I'm not sure.) ---So its probably very difficult to see any pattern to the boarding process.
So confusing....