Moses chose able men. It appears from Deuteronomy 1:13, that instead of selecting the men himself, which would have been an invidious task, Moses directed their nomination by the people, and only reserved to himself the investing them wit h their authority.
Heads over the people. From the time of their appointment, the "rulers" were not merely judges, but "heads" of their respective companies, with authority over them on the march, and command in the battle-field (Numbers 31:14).
Thus the organisation was at once civil and military.