Resident Country for determining Tax Treaty :

Resident Country is the country, which was your main country of work or education where you paid your taxes, before living in the United States.

This is mostly your Country of Citizenship. But in some cases, it can vary.

Example 1:

A student completes his education in India, and visits United States to study or work. His Country of Citizenship is India.

In this case, the resident country for Tax Treaty calculation is India. (same as the Country of Citizenship)

Example 2:

A student has his citizenship country as China. He has been working in Singapore for a few years, paying his taxes in Singapore, before visiting United States.

In this case, the Resident Country for Tax Treaty calculation is Singapore and not China.