From NecroWiki
Only Clerics and Warriors gain the benefit of this statistic.
Even if they wear armor with this statistic, Mages and Thieves will derive
zero benefit from it.
To understand this concept, you have to understand that defense bonus is
designed to reduce your chance of being hit. On items of type ARMOR, there
is a defense bonus value of varying amounts depending on the body location.
Armor rating works in a similar fashion, except that it helps reduce the
amount of hit points of damage that you take. For each point of armor
rating, the damage you take from each hit is reduced by one hit point. The
reduction comes from the added points of damage players get from their
damroll, and not from their actual weapon dice.
For example, suppose you have an armor rating of 3, and a mob hits you
three times for 20 hit points of damage per hit, you would only lose 51
hit points instead of 60. Each hit the mob made on you was reduced by
3 hit points.
