document sent to applicant?

Just check my I-485 status. My status changes to the following message. Anyone have the same experience? what's the document about?

Application Type: I485, APPLICATION TO REGISTER PERMANENT RESIDENCE OR TO ADJUST STATUS

Current Status: Document mailed to applicant.

On September 12, 2007 we mailed the document to the address we have on file. You should receive the new document within 30 days. If you do not, or if you move before you get it, call customer service.