请问各老师，收到I-485 APPROVAL NOTICE两周后，又收到新EMAIL。到底怎么回事？

今天又收到从USCIS发来的新EMAIL。

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

Current Status: Document mailed to applicant.

On May 15, 2008 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.

这里说的 DOCUMENT 到底是什么呀？？？？
有什么过来人说一下子？