דברי הימים א, פרק כ״ז, פסוק ג׳

The organization of the military leadership relied heavily on family loyalty and a clear chain of command. At the very top of this structure stood Jashobeam, whose authority was deeply rooted in his ancestry. He belonged to the family line of Perez, the son of Judah [ביאור שטיינזלץ]. This specific lineage was the primary reason for his senior status within the army. Because he was a descendant of Perez, he was a relative of King David. This close family tie led David to select him for such an elevated and trusted position [רש״י, מצודת דוד].

His responsibilities extended far beyond a standard temporary command. While his active duty fell during the first month of the year, his actual rank was much higher. He functioned as the supreme commander of the entire military structure. Every one of the twelve generals appointed to lead the army divisions throughout the different months of the year answered directly to him [רש״י, מצודת דוד, מלבי״ם].