Harry Bass John Sanford asked below about denominations of ancients. It just so happened that I had earlier received the following from Bill Metcalf, Head Curator at the ANS, on this very point. I'm new to collecting ancients.
Many coins have their denominations listed e.g. denarius, tetradrachm, or antoninianus.
Others are referred to as AE3 or AE4. The only thing I can think of is that through time the actual name is lost so it is given a label based upon base matel and size.