A superiority trial design aims to show that one treatment is clinically better than the other by demonstrating superiority of the test agent over placebo or an active treatment.
Regulatory agencies require a superiority trial when there is no licensed comparator available or the treatment effect of any licensed treatment is unknown or is considered questionable (e.g. acute self-limiting infections). For adjunctive treatments, the superiority design demonstrates a clinical effect of the adjunctive treatment vs. standard of care.
White Paper: Recommendations on the Conduct of Superiority and Organism-Specific Clinical Trials of Antibacterial Agents for the Treatment of Infections Caused by Drug-Resistant Bacterial Pathogens (Clinical Infectious Diseases, 2012)