Generally, a non-resident alien individual is engaged in trade or business within the US if he or she:

• You are considered to be engaged in a trade or business in the United States if you are temporarily present in the United States as a nonimmigrant on an “F,” “J,” “M,” or “Q” visa. The taxable part of any U.S. source scholarship or fellowship grant received by a nonimmigrant in “F,” “J,” “M,” or “Q” status is treated as effectively connected with a trade or business in the United States.
• If you are a member of a partnership that at any time during the tax year is engaged in a trade or business in the United States, you are considered to be engaged in a trade or business in the United States.
• You usually are engaged in a U.S. trade or business when you perform personal services in the United States.
• If you own and operate a business in the United States selling services, products, or merchandise, you are, with certain exceptions, engaged in a trade or business in the United States. For example, profit from the sale in the United States of inventory property purchased either in this country or in a foreign country is effectively connected trade or business income.
• Gains and losses from the sale or exchange of U.S. real property interests (whether or not they are capital assets) are taxed as if you are engaged in a trade or business in the United States. You must treat the gain or loss as effectively connected with that trade or business.
• Income from the rental of real property may be treated as ECI if the taxpayer elects to do so.

If a non-US Person is engaged in a US trade or business in the United States, all income, gain, or loss for the tax year that the non-US Person earns from sources within the US (other than certain investment income) is considered to be Effectively Connected Income (ECI) and subject to US tax at graduated rates unless rate under a tax treaty applies.  Generally, you can receive effectively connected income only if you are a nonresident alien engaged in a trade or business in the United States during the tax year. However, income you receive in another tax year from the sale or exchange of property, the performance of services, or any other transaction is treated as effectively connected in that year, if it would have been effectively connected in the year the transaction took place or you performed the services. This applies whether or not there is any connection between the income, and the trade or business being carried on in the United States, during the tax year.