Question/Suggestion on Pierce Mastery

Does this Mastery affect "Armor", as in the buff? Or does it affect "Armor Rating"?
Not all champs have an "Armor" buff but all champs have an "Armor Rating"
If it's only the buff, is it possible to change to go armor rating?
Not all champs have an "Armor" buff but all champs have an "Armor Rating"
If it's only the buff, is it possible to change to go armor rating?
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