好像找到了519里的相关描述，就是研究人员j签证算exempt，但是如果你在过去6年中
有2年exempt了后，就不算exempt，适用substantial presence test了。
You will not be an exempt individual as a teacher or trainee in
2013 if you were exempt as a teacher, trainee, or student for any part
of 2 of the 6 preceding calendar years.
Exempt individual. Do not count days for which you are an exempt
individual. The term “exempt individual” does not refer to
someoneexempt from U.S. tax, but to anyone in the following categories.
An individual temporarily present in the United States as a foreign
government-related individual under an “A” or “G” visa.A teacher or
trainee temporarily present in the United States under a “J” or “Q” visa
, who substantially complies with the requirements of the visa.
A student temporarily present in the United States under an “F,” “J,” “
M,” or “Q” visa, who substantially complies with the requirements of the
visa.A professional athlete temporarily in the United States to compete in a
charitable sports event.
