source of information :
http://www.path2usa.com/immigration/rela...94card.htm
Form I-94 or or the Crewman Landing Permit (Form I-95) also known as "Arrival Departure Record" is given to each foreign person enter into USA legally on any visa status. This shows the date you arrived in the United States and the date when you should leave the United states. This is a part of the form you fill when you enter into USA and officer staple a part of that form to your passport at POE.
This is an important document which every visitor should take good care and return while existing USA.
FAQ :
What should I do If I lost my I94 while in USA?
If your passport with your I-94 are lost or stolen, you must get them replaced immediately. There are a number of steps you need to take as follows:
Police Report
Go to the local police station and report your document(s) lost or stolen. If available, you will need to provide copies of the original documents. You will be issued a police report detailing the incident. Don't forget to make an extra copy of the report for your own records.
Request Replacement of a Lost/Stolen Arrival-Departure Record (Form I-94)
Replacement of a lost or stolen Form I-94 falls under the responsibility of Department of Homeland Security (DHS). To file for a replacement I-94, see Application for Replacement/Initial Nonimmigrant Arrival-Departure Document at the Department of Homeland Security (DHS), U.S. Citizenship and Immigration Services (USCIS) Website and see Arrival-Departure Record at the DHS, Customs and Border Protection (CBP) Website.
What should I do if I did not turn in my I-94 (Arrival / Departure Record) when I left the U.S.?
It is common and happened in many cases that I94 is not taken out by the airline checking counter at the airport while leaving the USA. You should always check whether airline checking person should take your I94, if not ask him/her to take it.
If for some reason you find that later once you already exit the USA and you send the the original I-94 to the following address:
ACS, Inc.
1084 South Laurel Road
London, Kentucky 40744
You must include with proof of departure. Suggested proof includes the arrival stamp in one's passport and the boarding pass from the flight. Always keep the copies of any documents submitted. The authorities warns that, without proof of the departure, there is no guarantee that the information will be entered into their system. This is logical, since otherwise, people could send the documentation and claim departure on a different date.
Check official wordings from CBP website on this Question.
Which I-94 Card Should I Hand Over while leaving USA ?
In some case when a visa holder has more then one I94, it become confusing which I94 should they give out while leaving USA.
People on working visa like H and L when they get their H or L visa extended or transferred they get the approval letter 797 with a new and extended I94. In such case First make a copy of this 797 and keep in your record, then take that I94 from the bottom of the approval letter and staple along with existing I94 which is in your passport. While exiting USA, must give all such I94 to the airline person who collects I94.
If you extend your stay you get a new I94 with approval. Instead of handing over the latest I-94 card upon departing the U.S., the individual either does not turn in any I-94 or the person turns in the initial, expired I-94. It is the latest original I-94 card approved by the USCIS showing that the individual was allowed to remain legally in the U.S. based on an approved extension that should be surrendered upon departure. Failure to properly return the I-94 card can create significant problems for return travel to the U.S., including a denial of entry for violation of prior status.
