What is the difference between phase 2 phase 2a and phase 2b clinical trials?

Phase 2a Clinical Trials are pilot studies done on a relatively smaller number of patients(100-300) in order to judge the efficacy and safety of the investigational drug.

Whereas Phase 2b Clinical trials are Pivotal studies, a well established controlled trails done on a larger number of patients in order to judge the efficacy and safety of the investigational drug.