It is a number that is issued to the US Permanent Residents (Green Card Holders), US citizens and those who hve work permit on their visas. This is a number, basically used for the taxation and identity purposes. Most of us, dont have it. If you dont have it or are not aware of it, just leave the space blank, no need to mention anything implying you dont have a SSN. You can apply for it once you go to US, if you get a work permit, it takes about 7-14 days in usual to arrive.
PS: SSN has no affect on ur application, its just used for identity purposes in US.


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