Spolia is the repurposed decorative sculpture used for new buildings. Much of the sculptural relief of the Arch of Constantine consists of reliefs and statues from previous monuments to Trajan, Hadrian, and Marcus Aurelius.   University of Pennsylvania archaeologist C. Brian Rose now suggests some of the spolia was also from an earlier unfinished monument to Diocletian:   Here’s an excellent video on the Arch of Constantine: Arch of Constantine (video) | La