In some recreation groups, the combat rules specify that certain pieces of armour are not vulnerable to certain weapons. For instance, in the SCA, plate armour is sometimes treated as arrow-proof, so a fighter wearing such armour is not "killed" or "injured" if the armour is hit by an arrow. However, other weapons (e.g. swords, cannonballs) are still effective. Depending on the group, these rules may be the norm, customary or one-offs for that particular war.
Late-period and renaissance armour was sometimes "proof" against pistol-balls. This was achieved by firing a pistol at the armour at close range which left a deep dent, but did not penetrate. Such "bullet-proofed" armour was highly prized.
As with many things nowadays and also during the past, these proofing marks could be, and were often, faked. Using chisels to make the mark is known as well as the common practise of using less gunpowder in the pistol than would normally have been used.