Sampada,

It's a tax ID number. If you have to go to school, then they need some kind of number that is assigned to you to identify (Just like SSN, but you can't get SSN since you can't work). For more info, visit http://www.irs.gov/businesses/small/international/article/0,,id=96696,00.html?portlet=1

Does anyone know how to get this number? I just went to Local IRS office and said they can't do it because it should be only for tax purpose which means the H4 holder was supposed to be working last year to file tax return, and apparently the H4 holder wasn't even here in US that time.