Help Understanding Building Armor: Iron Casting versus Arson

I don’t fully understand how building armor works and the difference between standard building armor and just building armor. Specifically, if I am trying to decide between buying Arson and Iron Casting for my infantry, and I want to calculate how much more damage I will be doing, is there any difference between how building armor will affect Arson and Iron Casting? Or will Arson always result in 1 additional damage being done to a building compared to iron casting? Thanks.

Building and Standard Building are two different armor classes. The difference is the Masonry and Architecture upgrades add Building armor, but not Standard Building armor, meaning units with an attack bonus against the Standard Building armor class won’t lose some bonus damage when the opponent researches Masonry or Architecture. Masonry and Architecture reduce the damage from Iron Casting by each also adding +1 Melee armor class to buildings. Therefore Arson is better than Iron Casting for attacking buildings because Arson won’t lose damage when the opponent researches Masonry or Architecture.