The armor value determines, how much damage a unit suffers when being attacked.
In Armies of Exigo, there is a highly complex system to allocate damage. On the one hand, there is the armor level that reduces any incoming damage, on the other hand, there are damage modifier that determine, how much damage is suffered from a certain damage type attack.
Invulnerable units have "invulnerable?" as the hitpoint value and cannot be targeted by any unit. All their damage modifiers are on 0 %.
For the exact modifiers of any single entity, please refer to the entitiy's wiki page.