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) | Late empire | Khan Academy   The inscription at the top of the arch honors the emperor’s victory over Maxentius. It is surrounded by sculptures of Dacian soldiers made for a monument commemorating victories of the emperor Trajan (r. A.D. 98–117).   A Monumental Imperial Biography - Archaeology Magazine