We have far more experts than I out there who know the Roman legions backwards and forwards - all I know is that the position of primum pilus ('first spear') was THE post of honor for a centurion. Apparently, Vorenus is the primum pilus (i.e., the best soldier) in the entire Thirteenth Legion. WHERE the position of Evocati comes in, I just found out today. Vorenus can be "evocati" because his legionary term has expired, but he comes back to the legion by choice. This permits him to move up beyond the position of Centurion.

Here is something about centurion ranks:

RedRampant.com

And here's something about evocati: Legion and Legionaries