TN as a non-immigrant status permits Canadian and Mexican citizens to be employed in the United States as a professional in certain fields defined by the NAFTA agreement (the NAFTA list of professional occupations).
The TN non-immigrant can enter the U.S. for up to three years at a time and renew it afterwards without maximum limits.
Because the TN is employment specific, the TN visa holder can only work for the intended employer. Note that 100% of the salary must be paid by UCSF.
For more information on TN status, visit U.S. Department of State web site.