For example: goods entering the United States via the Port of New York and New Jersey which are transferred to a distribution centre in New Jersey and eventually bought by consumers in Pennsylvania are listed as imports by the state of New Jersey.
More information can be found on the website of the US Census Bureau:
United States Census Bureau (2007). Guide to Foreign Trade Statistics. https://www.census.gov/foreign-trade/guide/index.html, 12 December 2018.