It is a three step unit and becomes a 6-6 if it loses a step. Similarly, the 10-7 units are four step units.
I find it hard to imagine building 6-6 shock armies. You need to spend at least 8 factors and at least four steps of units to get a two step unit worth 6 factors. It would have to be a very crucial attack that needs to get two more factors into a critical hex to make such a poor trade.