Where an artist receives repeated commissions from a donor it can be considered a clear approval of their work and displays trust that each work will be complementary to them and in line with their hopes and expectations. Sebastiano, therefore, must have been rated highly by Pope Clement VII who requested several different compositions at different points in his reign.
The Pope has a serious look in this portrait that is inline with most of the artist's portraits. He sits patiently on a chair in front of a very plain background which allows his attire to take centre stage. The presence of such a high ranking figure means very little other information is needed in this composition and Sebastiano allows the Pope to speak very much for himself.
Lighting is used by the artist here fairly modestly, just angling in from the left hand side of the painting. He would never use lighting as aggressively as the likes of Rembrandt or Caravaggio and prefered a much more subtle finish. His colour schemes too became somewhat dulled after he moved away from Venice.
Portrait paintings of Popes are, of course, particularly common. They also mark the point at which an artist has essentially reached the pinnacle in terms of donors. Once you have worked for the head of the Church, your reputation is essentially untouchable for the rest of your career. It may also gift the artist extra respect from future customers and enable him to have a greater artistic license.