A division of labor means that various people had various occupations. Of course, with a division of labor, there was a social hierarchy. Kings were at the top of the social hierarchy in Mesopotamia, obviously because they ruled certain areas.
Below them were priests. Religion was huge in ancient Mesopotamia, so it made sense for the priests to be in an upper class.
Then there were merchants who traded items to make their living. Bartering is always an important part of society, so the middle class consisted of mostly them. After that, there were farmers and laborers.
They are basically the lower end of merchants, because they too, offer their products, but benefit less from it. And at the bottom were slaves.