Does a visa guarantee entry into the United States and determine my legal status?
No, at the port of entry, a Homeland Security official must authorize the visitor’s admission and length of stay. At that time the Form I-94, Record of Arrival-Departure, which determines the permitted length of stay, is validated. Those visitors wishing to stay beyond the time indicated on their Form I-94 must apply for an extension using Form I-539, Application to Extend Status. The I-94 is what determines the validity of the visitor’s “status”, not the visa.