return service requested

I just received the document from INS for the authorization for parole of an alian into the Unites states. My question is: why is there a stamp of "return service requested" on the envolope?
What does it mean by "return service requested"?

Thanks a loooooooooot.

所有跟帖： 

• 就是告訴郵局，如果此人不在此地址，請將信件退回 -immigrate- ♂ (140 bytes) () 03/30/2005 postreply 19:46:58

• 拜謝 叩首 -playskool- ♀ (0 bytes) () 03/30/2005 postreply 19:51:26