ופקודיו אשר צוה ד' את משה
Ramban observes these three families of Leviim were Divinely designated, each family for its individual service. Furthermore, Ramban asserts that no Levi was permitted to go beyond his specific service. He was not to exchange his service or assist in an area for which he was not designated. Once Rabbi Yehoshua ben Chanania wanted to assist Rabbi Yochanan ben Gudgoda with the doors, and he was rebuked. The latter said to him, “Return! For you are already liable for death, since I am from those who attend to the gates, and you are of those who sing in the Bais…
