I-94是什么?

在收到的I20里有这么写的: You should present your I-20, passport, and completed white I-94 arrical record to the inspecting immigration officer at the port of entry when you arrive in the US.

不明白I-94是什么,有什么用?

还有啊,美国签证要提前一个月预约的吧,大使馆的签证是面试到开学4个月呢,还是从预约到开学4个月?

第一次申请,一切都在摸索中前进,请CDER们指点啊....