Auxiliary Shield Blazons

The latin term, clipeus is often used by modern historians to describe the flat oval shield carried by the Roman auxiliaries. However, this is not appropriate. The Roman applied the name clipeus to circular shields such as those carried by the ancient Greeks. Scutum was the general latin word for "shield," but there does not appear to be a specific word for the oval shields used by the auxilia.

The one monument that depcits the greatest number of shield design is Trajan's Column. The sculpture was created at the very hight of the Roman Empire and its army. The shields on the column represent a very specific time in Roman history, but their symbolism could be applied to anytime in early to mid imperial history. Lino Rossi, a modern historian, conveyed his ideas on the symbolism in his now out of print book, Trajan's Column and the Dacian Wars. I present a streamlined version of his concept here:

Numbers 1-4 were probably units granted Roman citizenship for valor. 4-8 were units raised from Roman citizen volunteers. He bases this hypothesis on the fact the these shields show Jupiter's eagle and lightning bolts—just as seen on the legionary scuta.
9-18 Are shields probably from units that were awarded the title torquata.

19-30. Various auxiliary designs. Note the rectangular shield carried by an auxiliary (30.). Rossi thought this had special significance, but it was probably just an error on the part of the sculptor.

The wings and thunderbolt design was almost never seen on the non-citizen auxiliary shields. The few times that it did appear could be explained by attributing those instance to auxiliaries who were awarded citizenship or those units who were not citizens, but served the emperor directly. (such as the emperor's bodyguard)

The wreath design is very common on auxiliary shields on Trajan's Column. A wreath (or cown) was awarded to a general who retuned from battle with victory. There were also a number of military decorations awarded as crowns. It can be guessed that whe this design was painted on a shield it meant that particular unit had recieved an award for some great victory.

References:
1. Greece and Rome at War, by Peter Connolly.
2. Rossi, Lino Trajan's Column and the Dacian Wars.