A Federal case is a criminal case that falls under the jurisdiction of a federal court. 

Federal courts are established under the U.S. Constitution to decide disputes involving the Constitution and laws passed by Congress. Federal courts only hear cases in which the United States is a party, cases involving violations of the U.S. Constitution or federal laws (under federal-question jurisdiction), cases between citizens of different states if the amount in controversy exceeds $75,000 (under diversity jurisdiction), and bankruptcy, copyright, patent, and maritime law cases.  

Federal cases are a primary source.

https://litigation.findlaw.com/legal-system/federal-vs-state-courts-key-differences.html

To find a federal case:

1. Begin on the Nexis Uni home page
2. Under "What are you Interested In?" select "Cases"
3. Under "Jurisdiction" select "Federal Cases"
4. Under "...about" type in your search terms
5. Under "All Dates" select a date range from drop down
6. Click "Search"
7. Narrow/limit your search using options in left panel