The drawing depicts Pope Sylvester I (reigned 314–335) in the sedia gestatoria, the portable papal throne. He is being carried out to meet the Emperor Constantine on the occasion of the Donation of Constantine, in which the pope was given territorial possessions and broad powers. The subject, which illustrated an important source of papal authority, was chosen for a fresco in the Vatican.
Source: Richard Lingner, "Procession of Pope Sylvester I," in Eye of the Beholder, edited by Alan Chong et al. (Boston: ISGM and Beacon Press, 2003): 79