What the player reads as Armour value printed by the client, is internally called resist_physical .

The Armour value is a sum of several values provided by race , class , dexterity , armour and probably a few other factors.
The higher the Armour value the better; in opposite to the Armour Class value.
The Armour Class provides that the player is not hit at all, while the Armour value helps to keep low the damages received.