Armor class is an alternate path of evolution some Digimon can take.
Originally, using a Digi-Armor egg, a Digimon could evolve into a special armor form unique to the combination of the Digi-Armor egg.
This is useful if there are any factors prohibiting normal evolution.
Eventually, however, Armor Digimon began to be treated the same way as champion class Digimon.
In general, if a Digimon used a Digi-Armor egg to evolve to the form, its considered Armor Body - while attaining it naturally makes it champion class.
Although usually rookie class evolve into Armor forms, other Digimon such as champion or Fusion classes can evolve into Armor forms.
Generally, Armor Digimon can eventually evolve into Ultimate class digimon, just like champion class digimon.