Nonresident income is usually considered to be any income you made in a state where you did not live. This includes income from working in Iowa as well as any income you received from Iowa sources.
For example, if you lived in Michigan but worked in Iowa for a few weeks, you would need to report the Iowa income on a nonresident Iowa return.
Also, if you received business income in Iowa while living in another state, then you would include any business income received in Iowa.
Dividend income or taxable interest income should only be reported as Iowa income if it comes from an Iowa trade, business, or profession.