Minecraft wiki entry for Attribute, associated text: multiply_base (amount % +/-, additive): Saved as operation 1. Multiplies the current value of the attribute by (1 + x), where x is the sum of the modifiers' amounts. For example, modifying an attribute with {Amount:2,Operation:1} and {Amount:4,Operation:1} with a Base of 3 results in 21 (3 * (1 + 2 + 4) = 21).