What are the major U.S. tax issues that apply to an inbound transaction?

The major U.S. tax issues that apply to an inbound transaction involve

1 ) whether the non - U.S. person has nexus in the United States ( is subject to U.S. taxation ),

2 ) whether the income earned by the non-U.S. person is from U.S. sources,

3 ) the type of U.S. source income earned ( income effectively connected with a U.S. trade or business ( ECI ) or income that is fixed, determinable , annual, or periodic ( FDAP ), and

4 ) whether a treaty applies to change the U.S. taxation of the transaction that otherwise would apply.

please provide detail of each one.