1, don't worry, you can fill it in again. The US visa form DS 160 is subject to the last submission. The American embassy confirmed this and later confirmed that it was possible. At the same time, there is a FAQ on the website of the US Embassy in China, which has a similar answer. You can have a look.

2. A visa is the endorsement and seal of the sovereign organ of a country on the passport or other travel documents held by its own or foreign citizens to indicate the procedures for allowing them to enter, exit or pass through its own territory. It can also be said that it is an endorsement certificate issued to them. Generally speaking, a visa is a document issued by a country's entry-exit administration department (such as immigration bureau or its embassy or consulate abroad) to express approval to foreign citizens.