A more thorough explanation:

A United States Commissioner is a judicial officer appointed by a U.S. district court to hear various pretrial matters in criminal cases. Their duties have been transferred to U.S. Magistrate Judges.

For example, a United States Commissioner may be responsible for conducting initial hearings, setting bail, and issuing search warrants. They may also oversee the arraignment process and make recommendations to the district court judge regarding plea agreements.

Overall, a United States Commissioner plays an important role in the criminal justice system by ensuring that pretrial matters are handled efficiently and fairly.