Plugin Help what does "damage value" mean?

What does "damage value" mean in this context? I'm confused as heck.. This plugin configuration thing seems unnecessarily difficult.. -_-

"

• denominations A key-value map, defining the item id of the actual item type to use as currency, and the value of the item. The keys may be either a single number for the item id, or two numbers in the format "id;damage", where "damage" is the damage aka data value of the item. The value can be a whole or fractional number denoting the value of a denomination. However the number of fractional digits in a currency value should not exceed the number defined as digits."

 

The example item they used was a creeper head:

"The following setup shows how to specify a currency with Lapis Lazuli as minor denomination with a value of 0.05, Skeleton Heads with a value of 10 and Creeper Heads with a value of 60:

denominations:
351;4: 0.05
397: 10
397;4: 60"

What the hell does the ;4 mean? Why is it called "damage?! Clearly it's not reffering to durability, because creeper heads have no durability.

So did some smart person just make "damage" mean something complety different than it's actual English definition to make people like me feel stupid? Because that's basically what it seems like.

 

@timtower Oh, so then why does the creeper head have a ;4 after it? It has no durabity more alternate blocks.

 

Okay, so heads are all one block type? 397? Then why doesn't the skeleton skull have a number after it?

 

Ahh, thanks!